JP2003015449A - Fixing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of the paper blister or toner blister of coated paper without decreasing fixing speed for plain paper. SOLUTION: This fixing device is provided with a coated paper discrimination means (59+C) for discriminating whether or not a recording sheet S passing through a fixing nip N is the coated paper having a coating layer M on its surface, a fixing temperature control means (47+59+C) for controlling the fixing temperature for the sheet S advancing to the fixing nip N to temperature lower than that of the plain paper when the discrimination means (59+C) discriminates that it is the coated paper or a nip pressing force control means (K) for controlling pressing force by which a press-contact member press- contacts with a thermal fixing member at the fixing nip to a value lower than that of the plain paper.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device for fixing an unfixed toner image on a recording sheet by heating and pressurizing the toner image, and more particularly to a fixing device capable of favorably fixing coated paper for obtaining image gloss. . The present invention can be applied to a roll pair type or belt type fixing device of an image information recording device such as a copying machine, a printer and a facsimile.

[0002]

2. Description of the Related Art Conventionally, plain paper (normal recording sheet) as shown in FIG. 14A has been used as a recording sheet for an image forming apparatus.
However, it has been used because it is easily available and inexpensive.
However, since the fiber layer G appears on the surface of this plain paper, the surface irregularities are large. In addition, the toner melted by the fixing device easily penetrates into the recording sheet, and uneven gloss after fixing is noticeable. Therefore, particularly in a color copying machine or the like which requires a high quality and high gloss image, the toner image surface has to be smoothly fixed. The fixing process further promotes the penetration of the toner into the recording sheet. Then, the toner is consumed to fill the irregularities, and the toner has to be replenished frequently. To compensate for this drawback, coated paper such as coated paper or art paper in which a coating layer M containing a resin agent is coated on the fiber layer G as shown in FIG. 14B is used. This is relatively expensive, but the surface has less unevenness and gloss, and the penetration of molten toner is small, so the uneven gloss of the image after fixing is less noticeable, and toner consumption and replenishment are also small, so especially in color copiers, etc. Often used in. As a fixing device that performs fixing processing so that the toner image surface after fixing is smooth,
Conventionally, the following technologies (J01) and (J02) are known.

(J01) Technology shown in FIG. 15 The roll pair type fixing device of FIG. 15 has a heater 01 inside and a relatively thick heat resistant elastic underlayer 02b covered with a top coat layer 02a. It has a heat fixing roll 02 and a pressure roll 03 having a relatively thin heat resistant elastic base layer 03b covered with a top coat layer 03a. Silicon oil is supplied to the surface of the heat fixing roll 02 from the oil supply device 04 in order to prevent the unfixed toner I on the recording sheet S from being transferred to the heat fixing roll 02. In the pressure contact portion (fixing nip N) of the pressure roll 03 and the heat fixing roll 02, since the relatively thick underlayer 02b is recessed, even a weak recording sheet S on which a large amount of toner I is placed like a color image is formed. After the fixing, it is possible to easily spontaneously peel off from the heat fixing roll 02 (self-stripping).

(J02) Technical Technique Shown in FIG. 16 FIG. 16 shows a belt nip type fixing device disclosed in JP-A-5-150679. In the fixing device shown in FIG. 16, elements given the same reference numerals as those in FIG. 15 are the same as those in FIG. 16A and 16B, the above-mentioned FIG.
The pressure roll 07 and the pressure assisting member 08 (or 08 ') that press the endless belt 06 against the lower surface of the heat fixing roll 02 are used in place of the pressure roll. The endless belt 06 and the heat fixing roll 0
The unfixed toner image I on the recording sheet S is fixed by receiving heat and pressure by the fixing nip N formed by the pressure contact of 2. In such a belt nip type fixing device, the time for heating by the length of the fixing nip N increases. Therefore, as compared with the case where the endless belt 06 is not used as shown in FIG. 15, a sufficient heat fixing time can be secured even if the conveying speed of the recording sheet S is increased, and conversely, if the conveying speed is the same, a larger amount of heat is generated. Therefore, it is suitable for fixing a color image. Further, since the pressure roll 07 is formed of a material such as stainless steel into a roll having a smaller diameter and being harder than the heat fixing roll 02, the pressing force distorts the surface of the heat fixing roll 01.
Self-stripping is possible.

(J03) Technique for prolonging heating and pressurizing time by decelerating fixing speed The following technique is known as a technique for decelerating fixing speed in order to prolong heating and pressurizing time. (A) JP-A-49-73141; a technique of reducing the fixing speed according to the thickness of recording paper. (B) Japanese Patent Application Laid-Open No. 51-78342; a technique of reducing the fixing speed in response to switching between a single color mode and a multicolor mode. (C) Japanese Patent Laid-Open No. 59-188673: A technique for switching between two speeds using an OHP button or the like. (D) Japanese Patent Application Laid-Open No. 60-143370 proposes means for detecting the type of recording paper and controlling the fixing speed and fixing temperature.

(J04) A technique for increasing the heating and pressurizing time by widening the fixing nip width. Techniques for widening the area of the fixing nip N are disclosed in JP-A-2-294579, JP-A-4-333083, and JP-A-4-33083.
There are No. 97187 and No. Kaikaidaira 2-58269.

[0007]

(Problems of (J01)) When the coated paper is fixed under the same fixing conditions (heating temperature, heating time, etc.) as plain paper in the roll pair type fixing device of (J01), However, there is a problem that the toner image on the coated paper is easily disturbed (hereinafter referred to as toner blister). In an even worse state, there is a problem in that the fiber layer G and the coating layer M are separated after passing through the fixing nip N and a paper blister in which the surface of the coated paper is swollen is generated. (Problem of (J02)) In the belt nip type fixing device of (J02), even when the coated paper is fixed under the same fixing conditions (heating temperature, heating time, etc.) as plain paper, the paper blister does not Although relatively unlikely to occur, there is a problem that toner blisters are likely to occur in a region only in the endless belt where the pressure contact force in the fixing nip N sharply decreases (for example, a region on the downstream side of the pressing assist member).

(Problem of (J03)) None of the techniques described as the method of lowering the fixing speed described the influence of the coated paper on the paper blister or the toner blister. Moreover, in these means,
No effect can be obtained on double-sided coated paper coated with a large amount of coating layer of about 5 gsm. (Problem of (J04)) Any of the techniques shown as a method of expanding the area of the fixing nip is intended to secure the fixing strength when a relatively thick recording paper is used, and is not necessarily thick. Not limited to coated papers.

The inventor considered the mechanism of generating the paper blister and toner blister described above as follows. That is, in the process of receiving fixing, the air in the recording paper expands and the moisture evaporates due to heating. In ordinary recording paper, these air and water easily escape through the voids between the fibers, but the coated paper Then, the coating layer M containing the resin is blocked and it is difficult to escape. Particularly, in the roll pair type fixing device, since the pressure contact force in the fixing nip N is relatively strong and abrupt heating is performed, there is no escape place for the expanded air and the evaporated water, and the air rapidly expanded at the outlet of the fixing nip N is used. The fiber layer G and the coating layer M are separated. On the other hand, in the belt nip type fixing device, when the pressures of the pressure assisting members 08 and 08 'and the pressure roll 07 are maintained at the same high pressure as in the case of plain paper, the pressure contact force sharply decreases in the region between them. Air and moisture in the recording paper come out in this area where the contact force is sharply reduced. At this time, since the toner is not pressed against the heat fixing roll, the disturbance of the toner image occurs due to the disturbance of the air and water.

Conventional measures against the above problems have been to not provide the coating layer M on the back surface of the recording paper, or to reduce the coating amount, which is usually about 15 gsm, to about 3 gsm. However, such improvement measures include drawbacks such as diminishing the above-mentioned advantages inherent to coated paper and inability to support double-sided copying. The present inventor has conducted research to prevent the occurrence of blisters in the coated paper. As a result, the fixing time is set longer than that in the case of plain paper, and heating, pressure fixing, and pressure application are performed at a time of 80 msec or more. We have found that the blisters can be prevented from occurring during fixing of coated paper by setting a lower temperature or performing preheating in advance, and performing heating and pressure fixing without rapid heating and pressure application. It was

In view of the above circumstances and examination results, the present invention has as its subject the contents described in (O01) below. (O01) To prevent the occurrence of paper blister or toner blister on coated paper without reducing the fixing speed of plain paper.

[0012]

The present invention devised to solve the above problems will now be described. The elements of the present invention are to facilitate correspondence with the elements of the embodiments described later. Note that the reference numerals of the elements of the embodiments are enclosed in parentheses. The combination of the signs in parentheses with a + sign indicates that a combination of a plurality of elements of the embodiments corresponds to an element of the present invention. The reason why the present invention is described in association with the reference numerals of the embodiments described later is to facilitate the understanding of the present invention and not to limit the scope of the present invention to the embodiments.

(First Invention) In order to solve the above problems,
The fixing device (F) of the first invention of the present application is provided with the following requirements, and the heat fixing member (41) arranged in the transport path (29) of the (Y01) recording sheet (S). And a pressure contact member (51, 51, which presses against the heat fixing member (41).
71, 71 '), (Y02) A fixing nip (N) is formed by the pressure contact area of the heat fixing member (41) and the pressure contact member (51, 71, 71'), and the fixing nip (N) passes through the fixing nip (N). The heat fixing member (41) for fixing the unfixed toner image (I) attached to the recording sheet (S), the pressure contact members (51, 71, 71 '), (Y03) pass through the fixing nip (N). Coated paper discriminating means (59) for discriminating whether or not the recording sheet (S) is a coated paper having a coating layer (M) on its surface, (Y05) said coated paper discriminating means (59)
If the recording sheet (S) is determined to be coated paper
Fixing temperature control means (47 + 59 + C) for controlling the fixing temperature of the sheet to a temperature lower than that of plain paper. In the first aspect of the invention, the recording sheet (S) passing through the fixing nip (N) is passed by an operation such as reducing the conveyance speed of the recording sheet (S) or increasing the width of the fixing nip (N). Nip passage time control means (49 + 59 + C; 47 + 49 + 59 + C; 69) for controlling the time to 80 msec or more
Can be provided.

(Second invention) In order to solve the above problems,
The fixing device (F) of the second invention of the present application is provided with the following requirements, and the heating fixing member (41) arranged in the transport path (29) of the (Y01) recording sheet (S) is characterized. And a pressure contact member (51, 51, which presses against the heat fixing member (41).
71, 71 '), (Y02) A fixing nip (N) is formed by the pressure contact area of the heat fixing member (41) and the pressure contact member (51, 71, 71'), and the fixing nip (N) passes through the fixing nip (N). The heat fixing member (41) for fixing the unfixed toner image (I) attached to the recording sheet (S), the pressure contact members (51, 71, 71 '), (Y03) pass through the fixing nip (N). Coated paper discriminating means (59) for discriminating whether or not the recording sheet (S) is a coated paper having a coating layer (M) on its surface, (Y06) said coated paper discriminating means (59)
The fixing nip (N) when it is determined that the paper is coated paper
In the nip pressing force control means (K), the pressing force of the pressure contact member (51, 71, 71 ') against the heat fixing member (41) is controlled to a value lower than that of plain paper.
The nip pressing force control means (K), in the fixing nip (N), determines that the pressure contact members (51, 71, 7) when the coated paper determination means (59) determines that the coated paper.
The pressing force of 1 ') against the heat fixing member (41) can be controlled to ² kg / cm ² or less.

(Explanation of Terms) In the present specification, the “heat fixing member” means a heat fixing roll, a heat fixing belt and the like. The "pressing member" means an endless belt, a pressure roll, a pressure assisting roll, a pressure assisting pad or the like that is in direct contact with the surface of the "heat fixing member".

[0016]

Next, the operation of the first invention of the present application having the above-mentioned features will be described. (Operation of the First Invention) In the fixing device (F) of the first invention having the above-mentioned features, the recording sheet (S) passing through the fixing nip (N) by the coated paper discriminating means (59) is When it is determined that the paper is coated paper, the fixing temperature control means (47 + 5)
9 + C) controls the fixing temperature of the recording sheet (S) to a temperature lower than that of plain paper. As a result, rapid expansion of air (per unit time) and evaporation of water from the recording sheet S during fixing at the fixing nip (N) can be suppressed. Therefore, the occurrence of blisters (paper blister, toner blister, etc.) on the coated paper can be prevented. In the first aspect of the invention, the recording sheet (S) passes through the fixing nip (N) by an operation such as reducing the conveyance speed of the recording sheet (S) or increasing the width of the fixing nip (N). 80mse time
nip passage time control means (49 + 59
When + C; 47 + 49 + 59 + C; 69) is provided, the coated paper can be satisfactorily fixed while preventing the occurrence of blisters (paper blister, toner blister, etc.) in the coated paper.

(Operation of Second Invention) In the fixing device (F) of the second invention having the above-mentioned characteristics, the coated paper discriminating means (5)
When 9) determines that the recording sheet (S) passing through the fixing nip (N) is coated paper, the nip pressing force control means (K) causes the pressure contact member (K) at the fixing nip (N). 51, 71, 71 ') are the heat fixing members (4
The pressing force for contacting 1) is controlled to a value lower than that for plain paper. As a result, the pressing force applied to the recording sheet (S) passing through the fixing nip (N) is made lower than that of plain paper, so that the inside of the recording sheet (S) passing through the fixing nip (N) is reduced. It can facilitate the movement and dissipation of air and water. Therefore, the occurrence of blisters (paper blisters, toner blisters, etc.) on the coated paper can be reduced. The nip pressing force control means (K) controls the pressing force at which the pressure contact members (51, 71, 71 ') are in pressure contact with the heat fixing member (41) in the fixing nip (N) to 2 kg / cm ² or less. This can prevent the occurrence of blisters (paper blister, toner blister, etc.) on the coated paper.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Next, embodiments of the present invention will be described. (Embodiment 1) Embodiment 1 of a fixing device (F) of the present invention is characterized in that the fixing device of the first invention or the second invention has the following requirements, (Y001) coating Preheating means (59 + 87 + C) for heating the coated paper before the paper reaches the fixing nip. As the preheating means, it is preferable to use preheating means (59 + 87 + C) which is non-contact with the coated paper and is heated by radiant heat or convection heat. The temperature of the coated paper when heated by preheating is preferably 60 ° C or higher.

(Operation of Embodiment 1) In Embodiment 1 of the fixing device (F) of the present invention having the above-mentioned structure, the coated paper discriminating means (59) passes through the fixing nip (N). When it is determined that the recording sheet (S) is coated paper, the preheating means (59 + 87 + C) raises the temperature of the recording sheet (S) entering the fixing nip (N) to 60 ° C. or more. Preheat to. Thereby, uniform preheating can be performed without adversely affecting the coating layer (M) on the surface of the recording sheet (S). When the preheating is performed with radiant heat or convective heat that is non-contact with the coated paper, it is possible to prevent the image on the coated paper from being disturbed.

(Embodiment 2) Fixing device (F) of the present invention
Embodiment 2 of the present invention is characterized in that the fixing device (F) of the first invention, the second invention or the first embodiment has the following requirements: (Y002) the coated paper discriminating means (5)
When 9) determines that the recording sheet (S) is coated paper, means for controlling the nip passage speed of the recording sheet (S) so that the nip passage time becomes longer than that of plain paper (nip passage speed). Nip passage time control means (49 + 59 + C) constituted by control means).

(Operation of Embodiment 2) In Embodiment 2 of the fixing device (F) of the present invention having the above-mentioned structure, the coated paper discriminating means (59) passes through the fixing nip (N). When it is determined that the recording sheet (S) to be processed is coated paper, the nip passage time control means (nip passage speed control means)
(49 + 59 + C) reduces the passage speed of the recording sheet (S) passing through the fixing nip (N), and makes the passage time of the recording sheet (S) 80 msec or more.
This makes it possible to prevent the occurrence of blisters even in a fixing device (F) which is relatively simple and has a narrow fixing nip (N) region, such as a roll pair type.

(Embodiment 3) Fixing device (F) of the present invention
Embodiment 3 of the present invention is characterized in that the fixing device according to any one of the first invention, the second invention, and the first or second embodiment has the following requirements. (Y003) The heat fixing member (41) ) Temperature sensor (47) for detecting the surface temperature of
(Y004) Fixing temperature control means (47 + 59) for lowering the surface temperature of the heating and fixing member when the coated paper determination means (59) determines that the recording sheet (S) is coated paper.
+ C).

(Operation of Third Embodiment) In the third embodiment of the fixing device (F) of the present invention having the above-described structure, the coated paper discriminating means (59) applies the recording sheet (S). When it is determined that the paper is engineering paper, the fixing temperature control means (47
+ 59 + C) lowers the surface temperature of the heat fixing member detected by the temperature sensor (47). This reduction of the surface temperature can be easily performed by reducing the heat generation amount of the heating means such as a heater. Accordingly, the blister prevention effect can be exhibited even in a fixing device (F) which is relatively simple and has a narrow fixing nip (N) region, such as a roll pair type. Further, by changing the fixing temperature and the fixing speed in combination, the blister prevention effect can be surely exhibited in a wider range of the recording sheet (S) and the unfixed toner (I).

(Fourth Embodiment) The fixing device (F) of the present invention
Embodiment 4 of the present invention is characterized in that the fixing device according to any one of the first invention, the second invention or the first embodiment has the following requirements: (Y005) the coated paper discriminating means ( 59), when the recording sheet (S) is determined to be coated paper, the nip passage time control means configured by means for increasing the nip area of the fixing nip (N) (nip area increasing means) (69).

(Operation of Embodiment 4) In Embodiment 4 of the fixing device (F) of the present invention having the above-mentioned structure, the coated paper discriminating means (59) applies the recording sheet (S). When it is determined that the paper is engineering paper, the nip passage time control means (nip area increasing means) (69) enlarges the width (area) of the fixing nip (N) and passes the recording sheet (S). Set the time to 80 msec or more. Thereby, the blistering prevention effect can be exhibited while performing the fixing process at a relatively high speed.

(Example) Next, an example (example) of an embodiment of the present invention will be described with reference to the drawings, but the present invention is not limited to the following examples. In order to facilitate understanding of the following description, orthogonal coordinate axes X axis, Y axis, and Z axis are defined in the directions of arrows X, Y, and Z that are orthogonal to each other in the drawings, and the arrow X direction is the front and the arrow Y direction is the Left, arrow Z
The direction is upward. In this case, the direction opposite to the X direction (front) (-X direction) is the rear, the direction opposite to the Y direction (left) is the right (-Y direction), and the direction opposite to the Z direction (upper) (-Z direction). Is below. In addition, the front (X direction) and the rear (-X direction)
It is called the front-back direction or the X-axis direction, including the left side (Y direction)
And the right side (-Y direction) are referred to as the left-right direction or the Y-axis direction, and the upper side (Z direction) and the lower side (-Z direction) are referred to as the up-down direction or the Z-axis direction. Furthermore, in the figure,
"○" in "○" means an arrow from the back of the paper to the front, and "X" in "○" means an arrow from the front to the back of the paper. Shall mean.

(Embodiment 1) Next, Embodiment 1 (example of the first embodiment) of the present invention will be described with reference to FIGS.
FIG. 1 is a configuration diagram of an entire image forming apparatus including a first embodiment of a fixing device according to the present invention. In FIG. 1, an image forming apparatus U is a digital copying machine A as an image forming apparatus main body having a platen glass (transparent document table) A1 on the upper surface.
And an automatic document feeder B which is detachably mounted on the platen glass A1. The automatic document feeder B sequentially takes out the originals stored in the original feed tray TR1, conveys them to the copy position on the platen glass A1, and discharges the copied originals to the original discharge tray TR2. It is configured.

The copying machine A has the platen glass A1.
An image input terminal IIT (hereinafter referred to as IIT) as an image reading section and an image output terminal IOT (hereinafter referred to as IOT) as an image recording operation section, which are sequentially arranged below the image pickup unit.

The IIT has an exposure scanning optical system 1 below the platen glass A1. The exposure scanning optical system 1 has an exposure lamp 2 for illuminating a document and a first mirror 3 mounted on a full rate carriage. Further, the exposure scanning optical system 1 has a second mirror 4 and a third mirror 5 mounted on a half rate carriage that moves at a speed half that of the full rate carriage. It has a supported imaging lens 6. The imaging lens 6 reflects from the original document on the platen glass A1 and further reflects the first, second,
It has a function of converging the reflected light of the original reflected by the three mirrors to a CCD (solid-state image pickup device). The CCD has a function of converting the reflected light of the document converged on the image pickup surface into an electric signal. Further, the IIT has an image data output unit 7 which receives an electric signal obtained by the CCD and outputs it as digital image data, and a user interface UI through which a user inputs and operates an operation command signal such as copy start. The user interface UI
Is a coated paper input button 59 (not shown) as a coated paper discriminating means for designating and inputting that the recording sheet S is coated paper.
In addition, the image forming apparatus U has a display unit (not shown) for displaying information regarding the current setting state of the image forming apparatus U.

The IOT is a laser drive circuit 8 for converting the image data output from the image data output section 7 into a laser drive signal, and an image writing unit for writing an image according to the laser drive signal output from the laser drive circuit 8. It has a laser writing device 9 for emitting the laser beam L. The laser beam L emitted from the laser writing device 9 is incident on the surface of the image carrier 11 at the latent image writing position Q1 set on the moving path of the surface of the image carrier 11 which rotates clockwise in FIG. It is supposed to do. A charger 12 is arranged along the surface of the image carrier 11 on the upstream side in the rotational direction of the latent image writing position Q1.

The image carrier 11 is provided downstream of the latent image writing position Q1 in the rotational direction along the surface of the image carrier 11.
A developing unit 13 for developing the electrostatic latent image formed on the surface into a toner image is arranged. The developing unit 13 has a red developing unit 13a and a black developing unit 13b. A transfer device 14 is arranged at a transfer position Q2 set further downstream of the developing unit 13. A cleaner unit 16 is arranged further downstream in the rotational direction than the transfer device 14. Below the transfer device 14, paper feed trays 21 to 25 are arranged in multiple stages. The paper feed tray 23 is 1 for double-sided copying or multiplex copying.
This is an intermediate tray used when the recording sheet (hereinafter referred to as a sheet) S after the second copying is circulated and retransmitted to the transfer position Q2. The sheet S of each of the sheet feeding trays 21 to 25 passes through the first sheet conveying path 26 and is transferred to the transfer position Q2.
It is supposed to be transported to. The transfer device 14 is arranged at the transfer position Q2, and the sheet S passing therethrough
It is configured to transfer the toner image on the surface of the image carrier 11 to. Image carrier 11 that has passed the transfer position Q2
After the toner remaining on the surface is collected by the cleaner unit 16, the surface is again uniformly charged by the charger 12.

At the end of the first sheet conveying path 26, the conveyed sheet S is temporarily stopped at the upstream side of the transfer position Q2 in the sheet conveying direction, and then the transfer position Q2 is reached.
Cash register 27 and cash register roll 28 for loading into a store
Are arranged. The sheet S having the toner image transferred at the transfer position Q2 is conveyed to the fixing position Q3 through the second sheet conveying path 29 connected to the sheet discharge tray TR3. A heating / pressurizing element of the fixing device F is disposed at the fixing position Q3, and the unfixed toner image I on the sheet S passing through the fixing position Q3 is fixed by heating / pressurization. A discharge roller 31 for discharging a sheet to the sheet discharge tray TR3 is provided on the second sheet conveying path 29 on the downstream side of the fixing position Q3.

A switching gate 32 is arranged upstream of the discharge roller 31 in the second sheet conveying path 29.
The switching gate 32 is used to switch the transport direction of the sheet S on the second sheet transport path 29 to the sheet circulation path 33 or the sheet discharge tray TR3. The sheet circulation path 33 is connected to the sheet inversion path 34 and the intermediate tray 23 via a switching gate 36.
The switching gate 36 directs the sheet S of the sheet circulation path 33 toward the sheet reversal path 34 when performing double-sided copying, and directly toward the intermediate tray 23 when performing multiplex copying. The sheet-shaped and comb-teeth-shaped Mylar gate 37 provided in the sheet inversion path 34 allows the sheet S to move downward due to elastic deformation when the passing sheet S is conveyed downward, 37
When the sheet S that has passed through is switched back and is conveyed upward, the sheet S is guided toward the intermediate tray 23. The sheet S once stored in the intermediate tray 23 is configured to be re-conveyed from the intermediate tray 23 to the transfer position Q2 by the first sheet conveying path 26.

Next, the fixing device F will be described. 2A and 2B are explanatory views of a belt nip type fixing device in Embodiment 1 of the present invention. FIG. 2A is a view showing a state of fixing a normal recording sheet, and FIG. 2B is a view showing a state of fixing coated paper. . In the figure, the sheet is represented by S, and the unfixed toner image transferred thereon is represented by I. In FIG. 2, the fixing device F
Has a heat fixing roll 41 as a heat fixing member. The core 42 of the heat fixing roll 41 has an outer diameter of 46 m.
It is an aluminum cylinder with m and an inner diameter of 42 mm. Core 42
On the surface of the base layer 43, a hardness of 45 ° (JIS-
The HTV silicone rubber of A) is directly coated to a thickness of 2 mm, and Viton rubber (registered trademark of DuPont) as a top coat layer 44 is further coated thereon to a thickness of 30 μm. An elastic body surface layer (43 + 44) is composed of the base layer 43 and the top coat layer 44. The outer diameter of the heat fixing roll 41 has the following value. Outer diameter of heat fixing roll 41 = 50.06 mm

The output of the halogen lamp 46, which is the heat source of the heat fixing roll 41, is 650 w, and the temperature sensor 47
A temperature controller (not shown) feedback-controls the output of the halogen lamp 46 based on the signal of (1), and the surface of the heat fixing roll 41 is 160 ° when the surface of the normal recording paper is fixed.
It is configured to be adjusted to C. Further, a mold release agent (amino-modified silicone oil having a viscosity of 300 cs) is supplied to the surface by an oil supply device 48 (see FIG. 1). The endless belt 51, which is a pressing member that presses against the heat fixing roll 41, is formed of a polyimide film with a thickness of 75 μm, a width of 300 mm, and a peripheral length of 188 mm. This endless belt 5
No. 1 is wound around the belt supporting rolls 52 and 53 and the pressing roll 54 with a tension of 98.06N. The belt supporting rolls 52 and 53 and the pressing roll 54 are made of stainless steel and have diameters of 18 mm, 18 mm and 23 mm, respectively.

The belt supporting rolls 52 and 53 and the pressing roll 54 are tapered so that the diameter of the central portion thereof is slightly larger than the diameter of the end portions thereof.
As a result, even if these rolls are bent due to the tension of the endless belt 51, the endless belt 51 is flat and can smoothly run without waviness. Of these rolls, the pressing roll 54
Is pressed toward the center of the heat fixing roll 41 by a constant pressing force (load) by the compression spring 56, whereby the endless belt 51 is moved to the heat fixing roll 41.
Being pressed against. This pressure contact structure will be described in detail later.

The compression spring 56 has one end (lower end in the drawing) fixed to the main body of the copying machine A and the other end (upper end).
Is fixed to a bearing holder of the rotating shaft (not shown) of the pressing roll 54 to apply a pressing force. 2A, when fixing a normal sheet, the heat fixing roll 41 is used.
The wrap angle of the endless belt 51 around the wrapping belt is 4
It is 5 °. The outer diameter of the heat fixing roll 41 is 50.
Since it is 06 mm, the pressure contact area of the heat fixing roll 41 and the endless belt 51 having the wrap angle of 45 ° (that is, the width of the fixing nip N where the fixing is performed).
Has the following value. N = 50.06 x 3.14 x 45/360 = 19.75 (mm)

Since the pressing roll 54 is made of stainless steel and is much harder than the top coat layer 44 of the heating and fixing roll 41, the heating and fixing roll 4 is protected by the pressing roll 54.
The endless belt 51 pressed toward the elastic surface layer (43 + 44) of No. 1 is in a state of biting into the surface of the heat fixing roll 41. In this state, the endless belt 51 is less likely to slip with respect to the heat fixing roll 41, and can travel at the same speed as the peripheral speed of the heat fixing roll 41. The fixing roll 4 is heated by the pressing roll 54.
Nip pressure P1 of the endless belt 51 pressed by 1
Is set to the following value in this embodiment. P1 = 4kg / cm2

In order to prevent the endless belt 51 from moving in the axial direction of the belt supporting rolls 52 and 53 and the pressing roll 54 and coming off from these rolls, the belt supporting roll 52 adjusts its axis direction. It is configured to be possible. That is, the belt support roll 52
The shaft (the front end of the paper surface) is supported so as to be slightly displaceable to the left and right in the paper surface as indicated by the arrow in FIG. The auxiliary pad 57, which is an auxiliary pressing member arranged on the upstream side of the pressing roll 54 in the traveling direction of the sheet S, includes a stainless steel support 57a having a width of 10 mm and a silicon sponge 57b (foam of silicone rubber) having a thickness of 2.5 mm. The body is covered with a polytetrafluoroethylene sheet 57c obtained by impregnating glass fibers with polytetrafluoroethylene to reduce the contact resistance with the endless belt 51. The auxiliary pad 57 is also pressed from the inside of the endless belt 51 toward the center of the heat fixing roll 41 by the compression spring 58. The nip pressure P2 of the endless belt 51 pressed against the heating and fixing roll 41 by the auxiliary pad 57 is set to the following value. P2 = 4kg / cm2

The heating / fixing roll 41 is driven by a drive motor 49 such as a step motor to produce a peripheral velocity Vo = 250 m.
Since it is rotated at m / sec, the speed of the endless belt 51 which is in pressure contact with the heating and fixing roll 41 is Vo = 25.
It is designed to run at 0 mm / sec. The peripheral speed of the heating and fixing roll 41 is the same as that of the drive motor 49.
It is determined according to the rotation speed of the drive motor and is calculated from the drive pulse of the drive motor 49 generated in the controller C. As mentioned above, N = 19.75 mm, Vo = 250 mm
The nip passage time t of the sheet S in the case of / sec is as follows. t = N / Vo = 19.75 / 250 = 79 msec

FIG. 3 is an explanatory view of the lap width varying mechanism of the belt nip type fixing device according to the first embodiment of the present invention. The detection signal indicating that the sheet S is coated paper is U
The coated paper input button 59 of I (user interface) is used to input to the copying machine controller C (hereinafter controller C). And the controller C
Is the heat fixing roll 4 of the endless belt 51.
1 to increase the wrapping angle (that is, the wrap angle) to increase the width of the fixing nip N.
Is raised from the position of FIG. 2A to the position of FIG. 2B.

In FIG. 3, the shaft 5 of the support roll 52 is shown.
2a and the pressure roll shaft 54a are held at a constant distance by an arm 61. The arm 61 is rotatably supported around the support roll shaft 52a,
The rotation shaft 54a of the pressing roll 54 is the compression spring 56.
Thus, the endless belt 51 is pressed against the surface of the heat fixing roll 41. A wrap width changing mechanism 62 for changing the wrap width (wrapping width) of the endless belt 51 on the surface of the heat fixing roll 41 is an arm 63 rotatably supported around the support roll shaft 52a.
have. A spring accommodating groove 63a is cut in the longitudinal direction at the tip of the arm 63, and a compression spring 64 is formed therein.
Is stored. The compression spring 64 is arranged so as to press the bearing holder 53b of the rotation shaft 53a of the support roll 53 slidably toward the tip of the arm 63, and the pressing force balances with the tension of the endless belt 51. There is. Further, in the vicinity of the support roll 52 on the arm 63, a gear 66 formed along an arc concentric with the support roll 52 is fixed, and the arc gear 66 meshes with a small gear 67.
The lap width varying mechanism 62 (63 to 64+) of the first embodiment is constituted by the elements indicated by the reference numerals 63 to 64 and 66 to 67.
66 to 67) are configured.

The pinion gear 67 is connected to a step motor 68 for controlling the lap width, and the step motor 6
8 is connected to the controller C. Then, the coated paper input button 59 of the UI (user interface)
When is input, the coated paper detection signal is input to the controller C. When the stepping motor 68 rotates forward and backward in response to the drive command from the controller C, the small gear 67 also rotates forward and backward. Accordingly, when the small gear 67 rotates, for example, in the clockwise direction (arrow direction in the drawing), the wrap width varying mechanism 62 rotates the arm 63 in the counterclockwise direction (arrow direction in the drawing) to support the support. The roll 53 is raised to increase the wrap angle. In this Example 1, the wrap angle is 45 ° for plain paper and 12 ° for coated paper.
(I.e., about 25%), and is set to 57 °. That is, the controller C
Is a required wrap angle of 57 ° when the sheet S is coated paper
And a table of corresponding signals for driving the step motor 68 are stored in a built-in memory, and when the coated paper detection signal is received, a required drive signal is read from the table and the step is performed via a drive circuit D3. It is adapted to be transmitted to the motor 68 and driven.

When the wrap angle is 57 °, the wrap width of the coated paper (that is, the width of the nip N) has the following value. N = 50.06 × 3.14 × 57/360 = 25.02 (mm) And N = 25.02 mm, Vo = 250 mm / sec
In the case of, the sheet n nipping passage time t is as follows. t = N / Vo = 25.02 / 250 = 0.1 sec = 100 (msec) As described above, from the elements 62, 68, C, and T, the nip passage time control means (nip of the first embodiment of the fixing device of the present invention (nip Area increasing means) 69 (62 + 68 + C + T) is configured.

(Operation of First Embodiment) Next, the operation of the first embodiment of the fixing device F of the present invention having the above-described structure will be described. In FIG. 1, in response to a copy command signal input from the user interface UI, the automatic document feeder B
Sequentially takes out the originals stored in the original feed tray TR1 and carries them into the copy position on the platen glass A1. In the IIT, the document is travel-exposed by the exposure lamp 2 for illuminating the document of the exposure scanning optical system 1 arranged immediately below the platen glass A1, and the reflected light is reflected by the first mirror 3 and the second mirror 4. Then, the light is reflected by the third mirror 5 and is focused on the CCD (solid-state image pickup device) through the imaging lens 6. The CCD converts the reflected light of the document converged on the image pickup surface into an electric signal and outputs the electric signal, and the image data output section 7 which receives the electric signal converts it into digital document image data and outputs it. The automatic document feeder B is
The copied document is taken out from the copy position and discharged to the document discharge tray TR2.

In the IOT, the laser drive circuit 8 which receives the above image data converts it into a laser drive signal and sends it to the laser writing device 9. In response to this signal, the laser writing device 9 makes the laser beam L for image writing incident on the latent image writing position Q1 on the surface of the image carrier 11. The surface of the image carrier 11 is uniformly charged in advance by the charger 12, but the electrostatic latent image corresponding to the original image is written by the irradiation of the laser beam L.
Next, the developing unit 13 arranged on the downstream side of the rotation of the image carrier 11 adheres toner to the electrostatic latent image and develops it into a toner image. At that time, the red developing unit 13a or the black developing unit 13b is switched and used as required. At the transfer position Q2 set further downstream, the transfer device 14 electrostatically attaches and transfers the toner image on the image carrier 11 onto the sheet S carried in by the registration roll 27. The residual toner on the surface of the image carrier 11 is scraped off by the cleaner unit 16 further downstream in the rotation direction.

The sheet S on which the toner image is transferred is conveyed to the fixing position Q3 through the second sheet conveying path 29. Here, the fixing device F fixes the unfixed toner image I on the sheet S (details will be described later), and when the fixing is finished, the fixing device F carries the sheet through the second sheet conveying path 29 to the downstream side. The discharge roller 31 discharges the sheet S to the sheet discharge tray TR3. On the other hand, in response to a copy command signal from the user interface UI, the sheet S is taken out from the corresponding tray (excluding the sheet feed tray 23) among the sheet feed trays 21 to 25, and passes through the first sheet conveyance path 26. It is conveyed until it comes into contact with the registration gate 27 at the end of the lever.
The registration gate 27 removes the temporarily stopped sheet S from
The toner image on the image carrier 11 is passed in synchronization with the arrival of the toner image, and is sent to the transfer position Q2 by the registration roll 28.

Except in the case of the one-sided single copy,
The switching gate 32 provided in the second sheet transporting path 29 is switched to transport the sheet S, which has been transferred on one side, to the sheet circulating path 33 below. Then, the lower switching gate 36 is operated to directly convey the sheet S to the intermediate tray 23 in the case of multiple copy. In the case of double-sided copying, the sheet S is once fed to the lower sheet reversing path 34 to be switched back, and the sheet S which has been turned upside down by the mylar gate 37 is sent to the intermediate tray 23. Intermediate tray 2
In No. 3, after temporarily stopping the sheet S at a predetermined position,
The sheet is again conveyed to the transfer position Q2 through the first sheet conveying path 26.

Next, referring to FIGS. 2 and 3, the fixing device F is described.
The action of will be described. First, at the fixing position Q3 shown in FIG.
When the sheet S conveyed from the right side of the sheet is not coated paper (normal recording paper), it is transferred onto the endless belt 51 of FIG. 2 and Vo = 2 by the drive motor 49.
A lap width (= 1) determined by the lap angle of 45 °, which is a pressure contact region between the heating and fixing roll 41 and the endless belt 51, which are rotationally moving at a peripheral speed of 50 mm / sec.
9.75 mm) is fed into the fixing nip N. Therefore, during the time t = 79 msec which passes through the nip N (= 19.75 mm), the heat of the heat fixing roll 41 having the surface temperature of 160 ° C. and the nip pressure between the heat fixing roll 41 and the endless belt 51. By this, the unfixed toner I is fixed on the sheet S.

That is, first, the heating and fixing roll 41 is heated by the halogen lamp 46 arranged at the center so that the surface is heated uniformly. The surface temperature is detected by the temperature sensor 47, is feedback-controlled via the controller C, and is maintained at 160 ° C. On the surface of the heat fixing roll 41, a thin film is formed by supplying a mold release agent of silicone oil from the oil supply device 48 (FIG. 1) on the upstream side, and this is a thin film, which is the heat fixing roll of the unfixed toner I. It suppresses metastasis to 41. The pressure required for the heat fixing is a constant pressure due to the endless belt 51 being in pressure contact with the wrapping angle (lap angle) of 45 ° around the heat fixing roll 41 with a tension of 98.06 N, and The pressing roll 54 and the auxiliary pad 57 are obtained by the pressing force that presses the inner surface of the endless belt 51.

Next, the case where the sheet S fed in is a coated paper will be described with reference to FIGS. When the coated paper input button 59 of the UI (user interface) is pressed, a detection signal indicating that the sheet S carried into the fixing nip N is coated paper is sent to the controller C.
In response to this input, the controller C reads the drive signal of the step motor 68 corresponding to the required wrap width when the sheet S is a coated paper from the table stored in the built-in memory, and drives it not shown. It transmits to the step motor 68 via a circuit. This signal causes the step motor 68 to rotate by a required angle, and the lap width varying mechanism 62 (63 to 64+) connected thereto.
66-67), when the small gear 67 rotates in the clockwise direction (arrow direction in the figure) as shown in FIG. 3, the arm 63 rotates in the counterclockwise direction (arrow direction in the figure), and The support roll 53 is raised from the position indicated by the broken line corresponding to the normal recording paper to the position indicated by the solid line to increase the wrap angle.

That is, the right end point R1 of the pressure contact area of the endless belt 51 with respect to the heating and fixing roll 41 on the ordinary recording paper moves to R2 as the support roll 53 moves up, and the width of the fixing nip N becomes R0 to. R1 to R0
Up to R2. Therefore, when the endless belt 51 is rotating at the same speed as in the case of normal recording paper, the passage time of the fixing nip N is (R0 to R2) /
(R0 to R1) times increase. As described above, the value of the fixing nip N and the nip passage time t are as follows. (For plain paper) Fixing nip N of (R0 to R1) N = 19.75 mm Fixing nip passage time t of (R0 to R1) of sheet S is t =
79 msec (in the case of coated paper) Fixing nip N of (R0 to R2) N = 25.02 mm Fixing nip passage time t of (R0 to R2) of sheet S t =
100 msec

As a result, the sheet S is allowed to pass through the fixing nip N for 100 seconds (80 msec or more), during which the air and moisture in the sheet S are slowly heated, and the coating layer M is added little by little per unit time. It becomes possible to permeate and leak. At the same time, the surface temperature of the heating and fixing roll 41 is controlled to 130 ° C., which is lower than the normal 160 ° C., so that the leak rate of air and moisture is further suppressed. When the input from the coated paper input button is released, the controller C outputs a signal for driving the step motor 68 from the table so that the wrap angle of 45 ° of the fixing nip N corresponding to normal recording paper can be obtained. After reading, the wrap angle is restored to the state of 57 ° corresponding to coated paper and 45 ° corresponding to plain paper (normal recording sheet) by the procedure reverse to the above. The above is the nip passage time control means (nip area increasing means) of the first embodiment.
It is the action of 69 (59 + 62 + 68 + C + T).

The relationship of blistering prevention by increasing the width of the fixing nip N in the first embodiment will be described with the knowledge obtained from the experimental results (FIGS. 4 to 5) of the inventor. In the experiment, the blister (paper blister, toner blister) is generated by changing the fixing roll temperature (that is, the surface temperature of the heating fixing member), the heating time (that is, the passing time of the fixing nip N), and the nip pressure (that is, the nip pressing force). The change in fixable temperature was investigated, and 90 gsm and 163 gsm double-side coated paper was used as the sheet S. FIG. 4 is a diagram showing the relationship among blister generation, fixing temperature, nip pressure, and heating time in the case of 90 gsm double-sided coated paper. FIG. 5 is a diagram showing the relationship among blister generation, fixing temperature, nip pressure, and heating time in the case of 163 gsm double-sided coated paper.

FIG. 4A shows a case where the nip pressure by the auxiliary pad 57 is 6 kg / cm 2 and the nip pressure is high and blisters are likely to occur. In FIG. 4A, the bent line at the uppermost portion on the lower right side indicates the limit of occurrence of paper blisters, and indicates that paper blisters occur when the fixing temperature is higher than this. The second polygonal line also indicates the limit of toner blister generation. Similarly, the third (bottom) polygonal line indicates that the fixing failure occurs at the minimum fixable temperature, that is, at the fixing roll temperature below this. Therefore, a region between the second broken line and the lowermost broken line is an appropriate region for fixing and preventing blisters. This area is called an available area in the figure. The above definitions are the same as in FIGS. 4 to 5. Figure 4A
Then, set the fixing roll temperature of coated paper to 160
When the temperature is ° C, it is difficult to fix the blister without occurrence of blister. However, the fixing roll temperature of coated paper is set to 1
When the heating time is 30 ° C., the usable area is extremely narrow when the heating time is 80 msec or less, but when the heating time is increased to 80 msec or more (right side in the drawing), the usable area expands vertically. Therefore, even if the fixing roll temperature changes to some extent, it is easy to always keep the fixing temperature and the fixing time within the usable area.

As shown in FIGS. 4 and 5, when the nip pressure is as small as about 2 kg / cm 2, the fixing time is 8
Even if it is 0 msec or less, it is relatively easy to avoid occurrence of toner blister. However, the nip pressure is 6
Fixing time is 8 when it is relatively large, such as kg / cm2.
If it is 0 msec or less, it becomes difficult to avoid occurrence of toner blister. Generally, when the nip pressure is low, toner blisters are unlikely to occur even when the fixing time is short, but as the nip pressure increases, the toner blisters easily occur when the fixing time is short. Then, regardless of the nip pressure, if the fixing time is 80 msec or more, the occurrence of toner blister can be reduced.

The heat fixing roll 4 in the first embodiment.
For a width of the fixing nip N of 19.6 mm when the peripheral speed of the surface is 250 mm / sec and the plain paper (wrap angle is 45 degrees), for example, the width of the fixing nip N is changed by operating the variable mechanism of the wrap angle. Increase by about 25%, the heating time is 100
It becomes msec (lap angle 57 °), and the fixing usable area shown in FIG. 4A is expanded. The above relationship is the same for other coated papers and nip pressures from FIG. 4B to FIG. 5B. However, as can be seen from FIGS. 4A and 5B, an appropriate position in the usable area (combination of fixing conditions)
Will be different.

(Embodiment 2) FIG. 6 is a schematic configuration diagram of a roll pair type fixing device (when a pair of large diameter rolls and a pair of small diameter rolls are used in combination) in Embodiment 2 of the present invention. In the description of the second embodiment, constituent elements corresponding to those of the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted. The second embodiment differs from the first embodiment in the following points. That is, as shown in FIG. 6, instead of the endless belt 51 as the pressure contact member, the belt support rolls 52 and 53, and the pressing roll 54 that contact the surface of the heat fixing roll 41, the heat fixing roll 41 functions as the pressure contact member. A pressure roll 71 having the same diameter as
It is pressed against the surface of the heat fixing roll 41 from below. The materials of the core 72, the base layer 73, and the top coat layer 74 in the constituent elements of the pressure roll 71 are the same as those of the core 42, the base layer 43, and the top coat layer 44 of the heat fixing roll 41, respectively. Is formed thinner than the base layer 43. As a result, the pressure roll 71 elastically deforms the heat fixing roll 41,
The fixing nip N is formed by slightly denting the surface to enable self-stripping.

Another difference is that the pressure roll 71 is not provided with the halogen lamp 46 provided inside the heat fixing roll 41. In the fixing nip N of the heating and fixing roll 41 and the pressure roll 71, the unfixed toner I on the sheet S is heated and pressed to be fixed. In addition,
The width of the fixing nip N can be adjusted by selecting the diameter of the heating fixing roll 41 and the pressure roll 71 or the material and thickness of the base layer 73 which is a constituent element thereof.

A sheet carry-in device 78 for carrying the sheet S into the fixing nip N is arranged on the right side (-Y direction) of the heating and fixing roll 41. The sheet carry-in device 78 includes a belt driving roll 79, a belt supporting roll 81, and a sheet conveying belt 82 stretched around them. The belt driving roll 79 and the belt supporting roll 81 rotate counterclockwise around the rotation shafts 79a and 81a, respectively, and carry in the sheet S on the sheet conveying belt 82 toward the fixing nip N. ing. The rotary shaft 79
A is configured to be rotationally driven by a drive device (not shown). The heat fixing roll 41 and the pressure roll 7
The combination of 1 and the sheet carry-in device 78 constitutes a large-diameter roll fixing section W.

In the second embodiment, another set of relatively small small-diameter roll fixing unit W'having the same structure as the large-diameter roll fixing unit W described above is arranged above.
Elements 41 ', 71' and 78 'of the small-diameter roll fixing section W'correspond to the heat fixing roll 41, the pressure roll 71, and the sheet carrying-in device 78, respectively. However, the cross-sectional dimensions of the heat fixing roll 41 'and the pressure roll 71' are smaller than those of the heat fixing roll 41 and the pressure roll 71. Therefore, the width of the fixing nip N'formed between the heat fixing roll 41 'and the pressure roll 71' of the small-diameter roll fixing unit W'is smaller than the width of the fixing nip N in the large-diameter roll fixing unit W. Has become. Further, the width of the fixing nip N ′ can be adjusted as in the fixing nip N. The widths of the fixing nips N and N ', the nip pressures P and P', and the fixing temperatures T and T'in the second embodiment are as follows, and the fixing temperatures T and T'are the same. N = 8mm Nip pressure P = 6kg / cm2 Fixing temperature T = 130 ° C N '= 4mm Nip pressure P' = 6kg / cm2 Fixing temperature T '= 160 ° C

A belt-type movable sheet conveying device 83 is arranged upstream of the belt supporting rolls 81 and 81 'of the large-diameter roll fixing unit W and the small-diameter roll fixing unit W'. The belt support roll 84 on the downstream side of 83 is supported by an arm 85, and is configured to be vertically adjustable by a solenoid 86 together with the downstream end of the arm 85. Then, the belt support roll 84 moves upward (the solenoid 86
Is off), the sheet conveyed by the sheet conveying device 83 is conveyed to the small-diameter roll fixing section W ′, and the belt supporting roll 84 moves downward (solenoid 8).
The sheet S is configured to be conveyed to the large-diameter roll fixing unit W when 6 is turned on. The second embodiment
Then, the sheet carry-in devices 78 and 78 'respectively form a part of the second transport path 29.

A coated paper detection signal from the coated paper input button 59 of the UI (user interface) is input to the controller C. The solenoid 86 is energized by a drive signal from the controller C which receives the signal. The solenoid 86 is turned on and off to adjust the vertical position of the belt support roll 84 of the belt type movable sheet conveying device 83. Above, reference numeral 59,
The elements 83 to 86, W, W ', C and the like constitute the nip passage time control means (nip area increasing means) 69 (59 + 83 to 86 + C + W + W') of the second embodiment of the fixing device of the present invention.

(Operation of Second Embodiment) Next, the operation of the second embodiment of the fixing device of the present invention having the above-described structure will be described. The operation of the second embodiment is different from the first embodiment in the following points, but is the same as the first embodiment in other points. In FIG. 6, when the sheet S sent from the movable sheet conveying device 83 on the right side is a normal recording sheet, the sheet is conveyed through the upper sheet conveying belt 82 'as shown at present. The fixing nip N'of the small-diameter roll fixing unit W '.
200 mm / s by the drive motor 49 '.
The unfixed toner I is fixed on the sheet S by being heated and pressed by the heating and fixing roll 41 ′ rotating at a peripheral speed of ec and the pressure roll 71 ′ pressed against the heating and fixing roll 41 ′. The sheet S which has been fixed is sent to the left downstream (not shown). The nip passage time, that is, the fixing time t'and the fixing temperature T'in this case are as follows. t '= 4 mm / 200 mm / sec = 20 msec T' = 160 ° C

Next, the case of coated paper will be described. The UI
When a coated paper detection signal is input from the coated paper input button 59 of (user interface), the detection signal is sent to the controller C. The controller C receives this input, outputs a drive command, and energizes the solenoid 86 via the drive circuit D4. The energized solenoid 86 moves the belt support roll 84 downward.

By lowering the belt support roll 84,
The sheet S from the movable sheet conveying device 83 is
Next, it is sent to the fixing nip N of the large-diameter roll fixing unit W via the sheet conveying belt 82 below, and is heated and pressed by the heat fixing roll 41 and the pressure roll 71 having a peripheral speed of 100 mm / sec. It is fixed. The nip passage time, that is, the fixing time t and the fixing temperature T in this case are as follows. t = 8 mm / 100 mm / sec = 80 msec T = 130 ° C

After passing through the fixing nip N, the sheet S is discharged in the same manner as in the small diameter roll fixing section W '. As a result, a lower fixing temperature of 130 ° C. and a longer fixing time 80 than those in the small-diameter roll fixing section W ′ are set.
It will be fixed for msec. When the coated paper detection signal from the coated paper input button 59 is released, the solenoid 86 is turned off and the belt support roll 84 is turned off.
And the sheet S from the movable sheet conveying device 83 is sent to the fixing nip N ′ of the small-diameter roll fixing section W ′ via the upper sheet conveying belt 82 ′, and corresponds to plain paper. You will receive heating for a short time. The above is the nip passage time control means (nip area increasing means) 69 (59 + 83 to 86 + C + W) of the second embodiment.
+ W ').

(Third Embodiment) Next, a third embodiment of the present invention will be described with reference to FIGS. FIG. 7 is an overall explanatory view of the fixing device according to the third embodiment of the present invention. FIG. 8 is a flowchart showing the fixing speed control of the fixing device in the third embodiment. In the third embodiment, constituent elements corresponding to those of the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted. The third embodiment differs from the first embodiment in the following points, but is configured similarly to the first embodiment in other points. That is, in the fixing device F of the third embodiment, instead of the endless belt 51 of the first embodiment, the elements 52 to 58 for pressing the endless belt 51 to the heating and fixing roll 41, and the lap width varying mechanism 62, pressure is applied. A roll 71 is provided. The nip passage time control is performed by the drive motor 4 of the heating and fixing roll 41.
It is configured to be performed by the nip passage speed control for controlling the rotation speed of No. 9.

The control parameters are set as follows depending on whether the sheet S is plain paper or coated paper. The widths of the nip pressure P and the nip N do not change between plain paper and coated paper. (For plain paper) Nip pressure P = 6 kg / cm2 N = 8 mm Peripheral speed Vo of the heat fixing roll 41 Vo = 250 mm / sec Fixing temperature T = 160 ° C (For coated paper) Nip pressure P = 6 kg / cm2 N = 8 mm, peripheral speed Vo of heating fixing roll 41 Vo = 100 mm / sec, fixing temperature T = 130 ° C.

When the parameters are set as described above,
The nip passage time t is as follows. (For plain paper) t = (8 mm) / (250 mm / sec) = 32 msec (For coated paper) t = (8 mm) / (100 mm / sec) = 80 msec

The coated paper detection signal from the coated paper input button 59 is input to the controller C. A program for executing the flowchart shown in FIG. 8 is incorporated in the controller C. In the flowchart of FIG. 8, there is a determination step ST1. In the case of normal recording paper, steps ST2 and ST3 of fixing at a normal fixing roll temperature and fixing speed are activated, and in the case of coated paper, a low fixing roll temperature and Steps ST4 and ST5 for fixing at a low fixing speed are operated.

(Operation of Third Embodiment) Next, the operation of the third embodiment of the fixing device of the present invention having the above-described structure will be described with reference to FIGS. 7 and 8. In step ST1 of FIG. 8, it is determined whether the fixing paper is coated paper. This judgment is made by the coated paper input button 5 on the UI (user interface).
It is determined by whether or not 9 is input. In the case of No (N), since it is plain paper, the halogen lamp 46 and the drive motor are set so that the fixing temperature T = 160 ° C. and the peripheral speed Vo of the heat fixing roll 41 is 250 mm / sec in step ST2. Activate 49.

When the coated paper input button 59 of the UI (user interface) is pressed, the answer is YES (Y) in step ST1. In this case, step ST
Go to 3. In step ST3, the control value of the heating temperature (surface temperature of the fixing roll) is set to 130 ° C. Then, the process proceeds to step ST4. In step ST4, the set value of the fixing speed (peripheral speed of the heat fixing roll 41) Vo is set to 10
Set to 0 mm / sec. Thus, the fixing process is performed on the coated paper in a proper fixing condition region. The above is the fixing temperature control means 47 + 59 + C and the nip passage time control means (nip passage speed control means) in the third embodiment.
It is the action of 49 + 59 + C.

(Embodiment 4) FIG. 9 is an overall explanatory view of a fixing device according to Embodiment 4 of the present invention. In the fixing speed control of the fixing device in the fourth embodiment, the same processing as the flowchart shown in FIG. 8 of the third embodiment is performed. In addition, this Example 4
The same reference numerals are given to the components corresponding to the components of the first embodiment, and the detailed description thereof will be omitted. Example 4 is different from Example 1 in the following points,
The other points are the same as those in the first embodiment. That is, in the fixing device F of the fourth embodiment, the wrap width varying mechanism 62 of the first embodiment is omitted. Therefore, in the fourth embodiment, when the fixing paper is a coated paper, the wrap width cannot be changed to extend the fixing time. Therefore, the rotation speed of the heat fixing roll 41 is decreased to extend the fixing time. It is configured. That is, the nip passage time control is performed by the nip passage speed control for controlling the rotation speed of the drive motor 49 of the heating and fixing roll 41.

In the fourth embodiment, the control parameters are set as follows depending on whether the sheet S is plain paper or coated paper. The nip pressures P and N do not change in the case of plain paper and coated paper. (For plain paper) Nip pressure P = 6 kg / cm2 N = 19.75 mm Peripheral speed Vo of the heat fixing roll 41 = 250 mm / sec Fixing temperature T = 160 ° C. (For coated paper) Nip pressure P = 6 kg / cm2 N = 19.75 mm Peripheral speed Vo of heating fixing roll 41 = 200 mm / sec Fixing temperature T = 130 ° C.

When the parameters are set as described above,
The nip passage time t is as follows. (For plain paper) t = (19.75 mm) / (250 mm / sec) = 79 msec (For coated paper) t = (19.75 mm) / (200 mm / sec) = 98.75 msec

The coated paper detection signal from the coated paper input button 59 is input to the controller C. A program for executing the flowchart shown in FIG. 8 is incorporated in the controller C. The program has a determination step ST1, and in the case of normal recording paper, steps ST2 and ST3 for fixing at a normal fixing roll temperature of 160 ° C. and a fixing speed of 250 mm / sec.
Is activated, and in the case of coated paper, a low fixing roll temperature of 130 °
Steps ST4 and ST5 for fixing at C and a low fixing speed of 200 mm / sec are operated. Therefore, the fourth embodiment has the same operation as the third embodiment.

(Fifth Embodiment) Next, a fifth embodiment of the present invention will be described with reference to FIG. FIG. 10 is a schematic diagram of a preheating device according to the fifth embodiment of the present invention. In the description of the fifth embodiment, constituent elements corresponding to those of the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted. This Example 5 is similar to Example 1 in the following points
However, in other respects, the configuration is similar to that of the first embodiment. In the fifth embodiment, the wrap width varying mechanism of the first embodiment is omitted. Then, the preheating device 87 is provided above the second transport path 29 on the upstream side of the fixing device F.
Are arranged.

The preheating device 87 comprises a housing 88 that covers the upper part and a heater 89 that is mounted immediately below the housing 88 with a slight gap. A plurality of the heaters 89 are arranged orthogonal to the carrying direction of the second carrying path 29, and are turned on / off with a predetermined input power in response to a signal from the controller C. Accordingly, when the sheet S conveyed on the second conveying path 29 is a coated paper, the sheet S is preheated. Further, the lower surface of the housing 88 is covered with a reflective metal surface, and is formed in a shape capable of efficiently reflecting the heat of the heater 89 and evenly preheating the surface of the sheet S. Further, the controller C has a simple program that replaces the program and the table of the second embodiment. The simple program is necessary to preheat the sheet S conveyed to the preheating device 87 to 60 ° C. or more when receiving a coating paper detection signal from the coating paper input button 59. The data of the operation timing, operation time, and heater input power of the heater 89 are stored. Then, when the simple program receives the coated paper detection signal, the data is taken out, converted into an operation signal of the heater 89, and the preheating device 8 is operated.
It is designed to be sent to 7. Reference numerals 59 and 8 above
The preheating means (59 + 87 + C) of Embodiment 5 of the fixing device of the present invention is constituted by the elements 7 and C.

(Operation of Embodiment 5) Next, the operation of Embodiment 5 of the fixing device of the present invention having the above-described structure will be described. The operation of the fifth embodiment is different from the second embodiment in the following points, but is the same as the second embodiment in other points. The UI
When the coated paper input button 59 of (user interface) is pressed and a coated paper detection signal is sent to the controller C, the controller C turns on the heater 89 and heats it at a predetermined heating temperature for a predetermined time. Then, the sheet S conveyed on the second conveying path 29 is preheated to 60 ° C. or higher. When the fixing paper is plain paper, the heater 89 remains off. The above is the operation of the preheating means (59 + 87 + C) of the fifth embodiment. As a result, air and moisture in the coated paper are released before fixing by preheating.

Note that the knowledge obtained from the experimental results of the inventor regarding the relationship of the blister prevention by the preheating in the fifth embodiment is as shown in FIG. That is, for two types of coated paper having coating amounts of 90 gsm and 163 gsm, the preheating temperature before fixing is 20 ° C to 90 ° C.
As a result of observing the state of occurrence of toner blister by changing the temperature between the above, it was found that the occurrence of toner blister was prevented when preheating at 60 ° C. or higher. Based on the above findings, the preheating temperature in Example 5 was set to 60.
The temperature was set above ° C.

(Sixth Embodiment) Next, a sixth embodiment of the present invention will be described with reference to FIGS. FIG. 12 is an overall configuration diagram of the nip pressing force control means according to the sixth embodiment of the present invention. 13A and 13B are detailed explanatory diagrams of the nip pressing force control means according to the sixth embodiment of the present invention. FIG. 13A shows a configuration of a pressure roll supporting device, and FIG. 13B shows a configuration of a pad supporting device. In the description of the fifth embodiment, constituent elements corresponding to those of the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

In FIG. 12, the pressing roll 54 is
The endless belt 51 is pressed toward the center of the heating and fixing roll 41 by the pressing force adjusted by the pressing roll support device 91, so that the endless belt 51 is heated.
It is pressed against 1. The auxiliary pad 57 is also pressed by the pad supporting device 92 from the inside of the endless belt 51 toward the center of the heat fixing roll 41. In FIG. 13A, the pressing roll supporting device 9
The outside of 1 is composed of a metal cylinder 93 and a thick bottom plate 94, and the bottom plate 94 is fixed by being screwed into a screw cut on the inner cylindrical surface of the cylinder 93.
Inside the cylinder 93, a compression spring 96 is housed in the upper part and a spring receiving piston 97 is housed in the lower part. A slider 101 is slidably supported on the cylinder 93. The slider 101 is connected to a bearing case 102 that rotatably supports the rotating shaft 54a of the pressing roll 54 using a connecting bolt 103.

A piston rod 98, which extends downward through the through hole 94a of the bottom plate 94, is connected to the lower surface of the spring receiving piston 97. A pressed plate 99 is fixed to the lower end of the piston rod 98. The lower surface of the pressed plate 99 is supported by the eccentric cam 104. The eccentric cam 104 moves the eccentric shaft 10 by the step motor 106.
It is designed to be rotated around 4a. The step motor 106 is driven by a drive circuit 107 connected to the controller C as shown in FIG. The pressure roll support device 91 is composed of the elements indicated by the reference numerals 93 to 103. The pressing roll support device 91 is disposed in the front and rear of the copying machine A (X direction).
It is arranged at both ends.

The pad support device 92 for the auxiliary pad 57 in FIG. 13B is also constructed and arranged in the same manner as the press roll support device 91 for the press roll 54. That is, the pad supporting device 92 is also configured by elements 93 'to 103' corresponding to the elements indicated by reference numerals 93 to 103 in FIG. 13A. Of the elements 93 'to 103', the pad connecting member 102 '
The other elements 93'-101 'and 103' differ from the bearing case 102 only in that
3 to 101 and 103. The step motor 106 'that drives the eccentric cam 104' of the pad support device 92 is driven by a drive circuit 107 'connected to the controller C.

In the sixth embodiment, the slider 101 shown in FIG. 13A is slidably accommodated in the cylinder 93 and at the same time rotatably accommodated therein. However, since the sliders 101 are arranged in a pair in the front-rear direction (X direction) and are connected by the pressure roll shaft 54 extending in the front-rear direction, they do not rotate in the cylinder 93. In addition,
In order to reliably prevent the rotation of the slider 101 in the cylinder 93, the cylinder 93 and the slider 101 are provided with
It is also possible to provide a guide groove along the axial direction and a plurality of guided pins slidably and non-rotatably engaged with the guide groove.

In FIG. 12, the coated paper detection signal from the coated paper input button 59 on the user interface UI is input to the controller C of the copying machine A. A temperature detection signal from the temperature sensor 47 is also input to the controller C. The controller C is a suitable pressing force value (pressure in the nip region 2 kg / c of the pressing roll supporting device 91 and the pad supporting device 92 corresponding to various (gsm) coated papers.
It has a memory storing a table of rotational position data of the step motors 106, 106 'for generating m2 or less, which will be described later). The step motors 106, 1 are read until the rotational position read from this table is reached.
By rotating 06 ', a predetermined pressing force can be generated.

Reference numerals 59, 104, 106 and 10 described above
The roll pressing force adjusting device J is constituted by the elements indicated by 7 and C. Similarly, reference numerals 59, 104 ', 10
The elements 6 ', 107' and C constitute a pad pressing force adjusting device J '. And reference numerals 91 and 9
Embodiment 6 of the fixing device of the present invention from the elements 2, J, J '
The nip pressing force control means K (91 + 92 + J + J ') is constructed. In the sixth embodiment, the control parameters are set as follows depending on whether the sheet S is plain paper or coated paper. Note that N does not change in the case of plain paper and coated paper. (In the case of plain paper) N = 19.75 mm Nip pressure P1 of the pressure roll 54 = 4 kg / cm2 Nip pressure P2 of the auxiliary pad 57 = 3 kg / cm2 Peripheral velocity Vo of the heat fixing roll 41 Vo = 250 mm / sec Fixing temperature T = 160 ° C (for coated paper) N = 19.75 mm Nip pressure P1 of pressing roll 54 = 4 kg / cm2 Nip pressure of auxiliary pad 57 P2 = 2 kg / cm2 Peripheral speed Vo of heating fixing roll 41 Vo = 200 mm / sec Fixing temperature T = 130 ° C

When the parameters are set as described above,
The nip passage time t is as follows. (For plain paper) t = (19.75 mm) / (250 mm / sec) = 79 msec (For coated paper) t = (19.75 mm) / (200 mm / sec) = 98.75 msec

The nip pressing force control means K (91 + 92 + J + J ') of the sixth embodiment is the pressing roll 54.
Although the pressing force on both the side and the auxiliary pad 57 side is adjustable, it is possible to adjust the pressing force by only one of the deviations.

(Operation of Sixth Embodiment) Next, the operation of the sixth embodiment of the fixing device of the present invention having the above-described structure will be described. Figure 1
2, when the fixing paper is plain paper, the rotational positions of the step motors 106 and 106 'are set to the pressed plates 99 and 99' on which the eccentric cams 104 and 104 'are mounted. Is increased to maintain the nip pressure P1 of the pressing roll 54 = 4 kg / cm2 and the nip pressure P2 of the auxiliary pad 57 = 3 kg / cm2.

When the coated paper input button 59 of the UI (user interface) is pressed, a coated paper detection signal is sent to the controller C. Upon receipt of this input, the controller C rotates the step motors 106, 106 'until the rotational position reaches a rotational position according to the rotational position data of the step motors 106, 106' stored in a table of a built-in memory. Let Then, the step motors 106, 106 'rotate to predetermined positions, respectively, and the eccentric cams 104, 10 connected thereto are connected.
4'is also rotated to a predetermined position. As a result, the eccentric cams 104, 104 'lower the pressed plates 99, 99' placed above, and the compression springs 96, 104 '.
Reduces 96 'compression.

As a result, the pressing force on the pressing roll 54 and the auxiliary pad 57 is reduced, and the nip pressure P1 of the pressing roll 54 is maintained at 4 kg / cm2 and the nip pressure of the auxiliary pad 57 is maintained at P2 = 2 kg / cm2. It As a result, it is possible to occupy a position of an appropriate fixing condition within a usable area in which the occurrence of blisters in the carried-in coated paper can be prevented and good fixing performance can be maintained. Next, when the coated paper input button 59 is turned off and the input of the coated paper detection data is cancelled, the step motors 106, 106 'corresponding to the general recording paper are appropriately selected from the table of the processing program. The rotational position data is read out, and the pressing force on the nip area is returned to the state of plain paper (normal recording sheet) in the same manner as the above-mentioned procedure. As can be seen from FIGS. 4 to 5, blisters are less likely to occur by reducing the nip pressure. Therefore, when the fixing paper is the coated paper, the fixing time is lengthened and the nip pressure is lowered. The sixth embodiment can reduce the occurrence of blisters in the coated paper. The above is the operation of the nip pressing force control means K (91 + 92 + J + J ') of the sixth embodiment.

In the sixth embodiment, two coated paper input buttons 59 are provided according to the coating amount of coated paper, and the set pressures of the nip pressures P1 and P2 are set according to the coating amount. It is possible to make them different.

(Modifications) Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-mentioned embodiments, but is within the scope of the gist of the present invention described in the claims. Thus, various changes can be made. A modified embodiment of the present invention is illustrated below. (H01) Instead of the non-contact type preheating means shown in the fifth embodiment, a heating roll or a heating plate that contacts and heats from the back surface (inside) of the endless belt that conveys the recording sheet is used, or the recording sheet is heated. It is possible to use a heating transfer roll (a device in which a heater is incorporated in the transfer roll) that directly contacts and heats the back surface. Furthermore, it is also possible to provide a recording sheet moisture prevention heater in the paper feed tray section. (H02) The coated paper discriminating means can use an automatic discriminating device such as surface gloss detection or surface roughness detection instead of the coated paper input button provided in the user interface shown in the first embodiment. Is. (H05) The driving system of the fixing device has been described as a system in which the heating and fixing roll is driven by a motor in all the embodiments, but instead of this, a system of driving a pressure roll below the heating and fixing roll (roll pair type, belt-nip) Of the endless belt support roll) or a type of driving the endless belt support roll by a motor (in the case of the belt-nip type or the belt-belt type). (H06) The oil supply device 48 can be replaced with another release agent application device or omitted.

[0096]

The above-described fixing device of the present invention can achieve the following effects. (E01) It is possible to prevent paper blister and toner blister on the image forming apparatus side even with a coated paper having a large coating amount or both sides coated. As a result, double-sided copying of coated paper is possible, and it is not necessary to fill the irregularities on the surface of the recording sheet with toner, so toner consumption is small and frequent toner supply is not necessary. This effect is particularly useful in color copying or the like, which requires high quality and high gloss images. (E02) If the coated paper is not rapidly heated and pressed during fixing, rapid expansion of air and evaporation of water from the recording sheet can be suppressed. Blister prevention can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an entire image processing apparatus including a first embodiment of a fixing device according to the present invention.

2A and 2B are explanatory views of a belt nip type fixing device in Embodiment 1 of the present invention, FIG. 2A shows a state in which a normal recording sheet is fixed, and FIG. 2B shows a state in which coated paper is fixed. FIG.

FIG. 3 is an explanatory diagram of a lap width varying mechanism of the belt nip type fixing device according to the first embodiment of the present invention.

FIG. 4 is a diagram showing the relationship between the occurrence of blisters in coated paper and the fixing temperature, nip pressure, and heating time (90).
gsm double coated paper).

FIG. 5 is a diagram showing a relationship between occurrence of blisters of coated paper and fixing temperature, nip pressure, and heating time (16).
3 gsm double-sided coated paper).

FIG. 6 is an explanatory diagram of a roll pair type fixing device (when a large diameter roll pair and a small diameter roll pair are used in combination) in Embodiment 2 of the present invention.

FIG. 7 is an overall explanatory diagram of a fixing device according to a third embodiment of the present invention.

FIG. 8 is a flowchart showing speed control of the fixing device according to the third embodiment of the present invention.

FIG. 9 is an overall explanatory diagram of a fixing device according to a fourth embodiment of the present invention.

FIG. 10 is a schematic diagram of a preheating device according to a fifth embodiment of the present invention.

FIG. 11 is an explanatory view of the operation of Example 5, showing the relationship between the occurrence of blisters on the coated paper and the preheating temperature.

FIG. 12 is an overall configuration diagram of a nip pressing force control means according to a sixth embodiment of the present invention.

13A and 13B are detailed explanatory diagrams of a nip pressing force control unit according to a sixth embodiment of the present invention, FIG. 13A showing a configuration of a pressure roll supporting device, and FIG. 13B showing a configuration of a pad supporting device.

FIG. 14 is an explanatory diagram of a recording sheet, and FIG.
4A is a sectional view of a normal recording sheet (plain paper), FIG. 14B
Shows a sectional view of the coated paper.

FIG. 15 is a schematic configuration diagram of a conventional roll-pair type fixing device.

FIG. 16 is a schematic configuration diagram of a conventional belt nip type fixing device, FIG. 16A shows a case where a pressure auxiliary roll is used as a pressure auxiliary member, and FIG. 16B shows a case where a pressure auxiliary pad is used.

[Explanation of symbols]

C ... Controller, F ... Fixing device, I ... Unfixed toner image, M ... Coating layer, N ... Fixing nip, S ... Recording sheet (sheet), 29 ... Conveying path (second sheet conveying path), 41 ... Heating Fixing member (heat fixing roll), 47 ... Temperature sensor, 4
9 ... Drive motor, 51 ... Endless belt, 54 ... Pressure contact member (pressure roll), 57 ... Pressure contact member (pressure auxiliary pad), 59 ... Coated paper discriminating means (Coated paper input button), 7
1 ... Pressure roll K (91 + 92 + J + J ') ... Nip pressing force control means,
(49 + 59 + C; 47 + 49 + 59 + C) ... Nip passage time control means (nip passage speed control means), (59+
87 + C) ... Preheating means 69 (59 + 62 + 68 + C + T; 59 + 84 + 86 +
C + W + W ') ... Nip passage time control means (nip area increasing means),

Claims (2)

[Claims] 1. A fixing device having the following requirements, (Y01) a heat fixing member arranged in a conveyance path of a recording sheet, and a pressure contact member which is in pressure contact with the heat fixing member,
(Y02) The heat fixing member and the pressure contact member, which form a fixing nip by the pressure contact area of the heat fixing member and the pressure contact member, and fix the unfixed toner image adhering to the recording sheet passing through the fixing nip. ) A coated paper discriminating means for discriminating whether or not the recording sheet passing through the fixing nip is a coated paper having a coating layer on its surface, (Y05) wherein the coated paper discriminating means is a coated paper Fixing temperature control means for controlling the fixing temperature of the recording sheet (S) to a lower temperature than that of plain paper when determined. 2. A fixing device having the following requirements: (Y01) a heat fixing member arranged in a conveyance path of a recording sheet; and a pressure contact member that is in pressure contact with the heat fixing member.
(Y02) The heat fixing member and the pressure contact member that form a fixing nip by the pressure contact area of the heat fixing member and the pressure contact member, and fix the unfixed toner image adhering to the recording sheet passing through the fixing nip. ) A coated paper discriminating means for discriminating whether or not the recording sheet passing through the fixing nip is a coated paper having a coating layer on its surface, (Y06) wherein the coated paper discriminating means is a coated paper A nip pressing force control means for controlling the pressing force of the pressure contact member to be in pressure contact with the heat fixing member in the fixing nip to a value lower than that in the case of plain paper when determined.