JP3541847B2 - Fixing device - Google Patents

Description

The present invention relates to a fixing device for fixing an unfixed toner image on a recording sheet by applying heat and pressure, and more particularly to a fixing device capable of satisfactorily fixing coated paper for obtaining gloss of an image.
INDUSTRIAL APPLICABILITY The present invention can be applied to a roll-pair or belt-type fixing device of an image information recording device such as a copying machine, a printer, and a facsimile.

Conventionally, plain paper (normal recording sheet) as shown in FIG. 14A has been used as a recording sheet for an image forming apparatus because it is easily available and inexpensive. However, in this plain paper, since the fiber layer G is exposed on the surface, the unevenness of the surface is large. In addition, there is a disadvantage that the toner melted by the fixing device easily penetrates the recording sheet, and gloss unevenness after fixing is conspicuous. Therefore, particularly in a color copying machine or the like that requires a high-quality and high-gloss image, the surface of the toner image must be fixed smoothly. By the fixing process, the penetration of the toner into the recording sheet is further promoted. Then, toner is consumed to fill the irregularities, and the toner must be replenished frequently.
In order 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 a fiber layer G as shown in FIG. 14B is used.
This is relatively expensive, but has little unevenness on the surface and is glossy, and the penetration of the molten toner is small, so that the gloss unevenness of the image after fixing is not conspicuous, and the consumption and replenishment of the toner is small. Often used in
As a fixing device that performs a fixing process so that the surface of a toner image after fixing becomes smooth, the following techniques (J01) and (J02) are conventionally known.

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

(J02) Technology 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, the components denoted by the same reference numerals as those in FIG. 15 are the same as those in FIG. 16A and 16B, a pressure roll 07 for pressing the endless belt 06 against the lower surface of the heat fixing roll 02 and a pressure auxiliary member 08 (or 08 ') are used instead of the pressure roll of FIG. The unfixed toner image I on the recording sheet S is fixed by receiving heat and pressure by a fixing nip N formed by pressing the endless belt 06 and the heat fixing roll 02.
In such a belt nip type fixing device, the heating time increases by the length of the fixing nip N. 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. This is suitable for fixing a color image.
Further, since the pressure roll 07 is formed of a material such as stainless steel into a harder and smaller-diameter roll than the heat fixing roll 02, the pressing force distorts the surface of the heat fixing roll 01 and enables self-stripping. I have.

(J03) Technology for Prolonging Heating and Pressing Time by Decreasing Fixing Speed The following technology is known as a technology for reducing the fixing speed in order to prolong the heating and pressurizing time.
(A) JP-A-49-73141; a technique for reducing the fixing speed according to the thickness of the recording paper.
(B) JP-A-51-78342; a technique for reducing the fixing speed in accordance with switching between a single color mode and a multicolor mode.
(C) Japanese Patent Application Laid-Open No. 59-188673; a technique for switching between two kinds of 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 the fixing temperature.

(J04) Technology for Extending the Heating and Pressing Time by Increasing the Fixing Nip Width The techniques for increasing the area of the fixing nip N include JP-A-2-294579, JP-A-4-333308, JP-A-4-97187, There is Japanese Utility Model No. 2-58269.
JP-A-5-150679 JP-A-49-73141 JP-A-51-78342 JP-A-59-188673 JP-A-60-143370 JP-A-2-294579 JP-A-4-3303083 JP-A-4-97187 Japanese Utility Model Publication No. 2-58269

(Problem of (J01) above)
In the roll-type fixing device (J01), when the coated paper is fixed under the same fixing conditions (heating temperature, heating time, etc.) as the plain paper, disturbance of the toner image on the coated paper (hereinafter, toner blister) is likely to occur. There is a problem. In a worse state, there is a problem 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 coated paper surface becomes swollen is generated.
(Problem of (J02) above)
Further, in the belt nip type fixing device of the above (J02), even though the coated paper is fixed under the same fixing conditions (heating temperature, heating time, etc.) as plain paper, paper blisters are relatively unlikely to occur. There is a problem that toner blisters are likely to occur in a region of only the endless belt (for example, a region on the downstream side of the pressing auxiliary member) where the pressure contact force in N rapidly decreases.

(Problem of (J03) above)
None of the techniques described as a method of lowering the fixing speed describes the effect of coated paper on paper blisters or toner blisters. In addition, these methods cannot provide an effect on double-sided coated paper coated with a large amount of a coating layer of about 15 gsm.
(Problem of (J04) above)
Any of the techniques described as a method for expanding the area of the fixing nip is intended to secure the fixing strength when a relatively thick recording paper is used, and does not necessarily correspond to a coated paper that is not always thick. .

The inventors considered the mechanism of generating the paper blister and the toner blister described above as follows. That is, during the fixing process, heating causes expansion of air and moisture in the recording paper due to heating, and in the case of ordinary recording paper, these air and moisture can easily escape through the gaps between the fibers. In this case, it is difficult to escape because of the coating layer M containing the resin.
In particular, in the roll pair type fixing device, since the pressing force at the fixing nip N is relatively strong and rapid heating is performed, there is no escape area for the expanded air or evaporated water, and the air rapidly expanded at the exit of the fixing nip N 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 decreases sharply in a region therebetween. Air and moisture in the recording paper come out in the area where the pressing force is sharply reduced. At this time, since the toner is not pressed against the heat fixing roll, the toner image is disturbed due to the disturbance of the air or moisture.

Conventional measures for solving the above-mentioned problem include not providing the coating layer M on the back surface of the recording paper or reducing the coating amount, which is usually about 15 gsm, to about 3 gsm. However, such an improvement has disadvantages such as diminishing the advantages inherent in coated paper and making it impossible to cope with double-sided copying.
The present inventor conducted a study to prevent the occurrence of blisters in the coated paper, and found that the fixing time was increased as compared with the case of plain paper, and heating, pressure fixing was performed in a time of 80 msec or more, and the pressing force was reduced. By setting the temperature lower or performing preliminary heating in advance, if heating and pressing and fixing are performed without performing rapid heating and pressing, it is found that the occurrence of the blister at the time of fixing coated paper can be prevented. Was.

The present invention has been made in view of the above circumstances and examination results, and has an object of the following content (O01).
(O01) To prevent paper blisters or toner blisters on coated paper without lowering the fixing speed of plain paper.

Next, the present invention devised to solve the above problem will be described. Elements of the present invention are indicated by parentheses in the elements of the embodiment in order to facilitate correspondence with the elements of the embodiments described later. Add the items enclosed in. A sign in parentheses connected by a + sign indicates that a combination of a plurality of elements of the embodiment corresponds to an element of the present invention.
The reason why the present invention is described in correspondence with the reference numerals of the embodiments described below is to facilitate 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-mentioned problems, a fixing device (F) according to a first invention of the present application has the following requirements.
(Y01) a heating / fixing member (41) disposed on the recording sheet (S) conveyance path (29) and pressing members (51, 71, 71 ') pressing against the heating / fixing member (41);
(Y02) A fixing nip (N) is formed by a pressure contact area of the heat fixing member (41) and the pressure contact members (51, 71, 71 '), and the recording sheet (S) passing through the fixing nip (N) is formed on the recording sheet (S). The heat-fixing member (41) for fixing the adhered unfixed toner image (I) and the pressing members (51, 71, 71 ');
(Y03) coated paper discriminating means (59) for discriminating whether or not the recording sheet (S) passing through the fixing nip (N) is a coated paper having a coating layer (M) on its surface;
(Y04) The speed of the recording sheet (S) passing through the fixing nip (N) is controlled to a value lower than that of plain paper when the coated paper discriminating means (59) determines that the paper is coated paper. Nip passing speed control means (49 + 59 + C).
In the first invention, the following requirement (A05) can be provided.
(Y05) Fixing temperature control means (47 + 59 + C) for controlling the fixing temperature of the recording sheet (S) to a lower temperature than that for plain paper when the coated paper discriminating means (59) determines that the paper is coated paper. ).
Further, in the first invention, the following requirement (A06) can be provided.
(Y06) When the coated paper discriminating means (59) determines that the paper is coated paper, the pressing members (51, 71, 71 ') are connected to the heat fixing member (41) in the fixing nip (N). Nip pressing force control means (K) for controlling the pressing force to be pressed to a value lower than that of plain paper.

(Second invention)
In order to solve the above problems, a fixing device (F) according to a second invention of the present application has the following requirements.
(Y01) a heating / fixing member (41) disposed on the recording sheet (S) conveyance path (29) and pressing members (51, 71, 71 ') pressing against the heating / fixing member (41);
(Y02) A fixing nip (N) is formed by a pressure contact area of the heat fixing member (41) and the pressure contact members (51, 71, 71 '), and the recording sheet (S) passing through the fixing nip (N) is formed on the recording sheet (S). The heat-fixing member (41) for fixing the adhered unfixed toner image (I) and the pressing members (51, 71, 71 ');
(Y03) coated paper discriminating means (59) for discriminating whether or not the recording sheet (S) passing through the fixing nip (N) is a coated paper having a coating layer (M) on its surface;
(Y07) When the coated paper discriminating means (59) determines that the paper is coated paper, the fixing nip passage time of the recording sheet (S) passing through the fixing nip (N) is longer than that of plain paper. (49 + 59 + C; 47 + 49 + 59 + C; 69).
In the second invention, the following requirement (Y05) can be provided.
(Y05) Fixing temperature control means (47 + 59 + C) for controlling the fixing temperature of the recording sheet (S) to a lower temperature than that for plain paper when the coated paper discriminating means (59) determines that the paper is coated paper. ).

(Explanation of terms)
In this specification, the “heat fixing member” means a heat fixing roll, a heat fixing belt, and the like. Further, the “pressing member” means an endless belt, a pressure roll, a pressure auxiliary roll, a pressure auxiliary pad, etc., which are in direct contact with the surface of the “heat fixing member”.

Next, the operation of the first invention of the present application having the above-described features will be described.
(Operation of the first invention)
In the fixing device (F) of the first invention having the above-described features, the coated paper discriminating means (59) determines that the recording sheet (S) passing through the fixing nip (N) is coated paper. In this case, when the coated paper determining means (59) determines that the paper is coated paper, the nip passing speed control means (49 + 59 + C) determines the speed of the recording sheet (S) passing through the fixing nip (N). Controlling to a lower value than in the case of plain paper By doing so, it is possible to suppress rapid expansion of air (per unit time) and evaporation of moisture from the recording sheet S during fixing in the fixing nip (N). Therefore, generation of blisters (paper blisters, toner blisters, etc.) on the coated paper can be prevented.
In the first aspect, a nip passing time control unit that controls a passing time of the recording sheet (S) passing through the fixing nip (N) to 80 msec or more by an operation such as increasing a width of the fixing nip (N). When (49 + 59 + C; 47 + 49 + 59 + C; 69) is provided, it is possible to satisfactorily fix the coated paper while preventing the occurrence of blisters (paper blister, toner blister, etc.) on the coated paper.
In the first aspect of the present invention, in the fixing device having the requirement (A05), when the coated paper discriminating means (59) determines that the paper is coated paper, the fixing temperature control means (47 + 59 + C) controls the recording. The fixing temperature of the sheet (S) is controlled to be lower than that of plain paper. In this case, the coated paper can be fixed more favorably while the generation of blisters (paper blister, toner blister, etc.) of the coated paper is further prevented.
In the first aspect of the present invention, in the fixing device having the requirement (A06), when the coated paper discriminating means (59) determines that the paper is a coated paper, the nip pressing force control means (K) In the fixing nip (N), the pressing force at which the pressure contact members (51, 71, 71 ') are pressed against the heat fixing member (41) is controlled to a lower value than in the case of plain paper. In this case, the coated paper can be fixed more favorably while the generation of blisters (paper blister, toner blister, etc.) of the coated paper is further prevented.

(Operation of the second invention)
In the fixing device (F) of the second invention having the above-described features, the coated paper discriminating means (59) determines that the recording sheet (S) passing through the fixing nip (N) is a coated paper. In this case, the nip passing time control means (49 + 59 + C; 47 + 49 + 59 + C; 69) makes the fixing nip passing time of the recording sheet (S) passing through the fixing nip (N) longer than that of plain paper.
This can prevent the occurrence of blisters even in a fixing device (F) having a relatively simple fixing nip (N) region such as a roll-pair type.
In the second aspect of the present invention, in the fixing device having the requirement (A05), when the coated paper discriminating means (59) determines that the paper is coated paper, the fixing temperature control means (47 + 59 + C) controls the recording. The fixing temperature of the sheet (S) is controlled to be lower than that of plain paper. In this case, the coated paper can be fixed more favorably while the generation of blisters (paper blister, toner blister, etc.) of the coated paper is further prevented.

(Form of the present invention)
Next, an embodiment of the present invention will be described.
(Embodiment 1)
Form 1 of the fixing device (F) of the present invention is characterized in that the fixing device of the first or second invention has the following requirements.
(Y001) Preheating means (59 + 87 + C) for heating the coated paper before the coated paper reaches the fixing nip.
As the preheating means, it is preferable to use a preheating means (59 + 87 + C) for heating by radiant heat or convective heat which is not in contact with the coated paper. Further, the temperature of the coated paper when heated by preheating is preferably set to 60 ° C. or higher.

(Operation of Embodiment 1 of the Invention)
In the first embodiment of the fixing device (F) of the present invention having the above-described configuration, the recording sheet (S) passing through the fixing nip (N) by the coated paper discriminating means (59) is a coated paper. If it is determined that the recording sheet (S) enters the fixing nip (N), the preheating means (59 + 87 + C) preheats the temperature of the recording sheet (S) to 60 ° C. or more. 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 by radiant heat or convective heat without contacting the coated paper, it is possible to prevent the image on the coated paper from being disturbed.

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

(Operation of Embodiment 2 of the Invention)
In the fixing device (F) according to the second aspect of the present invention having the above-described configuration, the coated paper discriminating means (59) determines that the recording sheet (S) passing through the fixing nip (N) is coated paper. If it is determined, the nip passing time control means (nip passing speed control means) (49 + 59 + C) reduces the passing speed of the recording sheet (S) passing through the fixing nip (N), thereby reducing the recording sheet (S). Is set to 80 msec or more. This can prevent the occurrence of blisters even in a fixing device (F) having a relatively simple fixing nip (N) region such as a roll-pair type.

(Embodiment 3)
A third aspect of the fixing device (F) 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 aspect of the invention has the following requirements.
(Y003) a temperature sensor (47) for detecting a surface temperature of the heat fixing member (41);
(Y004) Fixing temperature control means (47 + 59 + C) for lowering the surface temperature of the heat fixing member when the coated paper discriminating means (59) determines that the recording sheet (S) is coated paper.

(Operation of Embodiment 3 of the Invention)
In Embodiment 3 of the fixing device (F) of the present invention having the above-described configuration, when the coated paper discriminating means (59) determines that the recording sheet (S) is coated paper, the fixing temperature is determined. The control means (47 + 59 + C) reduces the surface temperature of the heat fixing member detected by the temperature sensor (47). This reduction in surface temperature can be easily performed by reducing the amount of heat generated by a heating means such as a heater.
Thereby, even in a fixing device (F) having a relatively simple fixing nip (N) region such as a roll-pair type, a blister prevention effect can be exhibited. In addition, by changing the fixing temperature and the fixing speed in combination, the blister prevention effect can be surely exerted over a wider range of the recording sheet (S) and the unfixed toner (I).

(Embodiment 4)
Form 4 of the fixing device (F) according to the present invention is characterized in that the fixing device according to any one of the second and the second aspects of the present invention has the following requirements.
(Y005) Means for increasing the nip area of the fixing nip (N) when the coated paper determining means (59) determines that the recording sheet (S) is coated paper (nip area increasing means) The nip passage time control means (69).

(Operation of Embodiment 4 of the Invention)
In the fixing device (F) according to the fourth aspect of the present invention having the above-described configuration, when the coated paper discriminating means (59) determines that the recording sheet (S) is coated paper, the nip passage is performed. The time control means (nip area increasing means) (69) enlarges the width (area) of the fixing nip (N) to make the passage time of the recording sheet (S) 80 msec or more. Thereby, the blister prevention effect can be exhibited while performing the fixing process at a relatively high speed.

The above-described fixing device of the present invention can provide the following effects.
(E01) Paper blisters and toner blisters can be prevented on the image forming apparatus side even on a coated paper having a large coating amount or a double-sided coating. As a result, it is possible to cope with both-sided copying of coated paper, and it is not necessary to fill irregularities on the surface of the recording sheet with toner, so that toner consumption is reduced and it is not necessary to frequently supply toner. This effect is particularly useful in color copying or the like that requires a high-quality and high-gloss image.
(E02) If rapid heating and pressurization of the coated paper are not performed at the time of fixing, rapid expansion of air and evaporation of water from the recording sheet can be suppressed. Blister prevention can be realized.

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 example.
In order to facilitate understanding of the following description, orthogonal coordinate axes X axis, Y axis, and Z axis are defined in directions of arrows X, Y, and Z orthogonal to each other in the drawings, and the arrow X direction is forward, and the arrow Y direction is The left, arrow Z direction is the upper side. In this case, the direction opposite to the X direction (forward) (−X direction) is backward, the direction opposite to the Y direction (left) (−Y direction) is right, and the direction Z (upward) is opposite (−Z direction). Is below.
Also referred to as the front-rear direction or the X-axis direction including the front (X direction) and the rear (-X direction), the left-right direction or the Y-axis direction including the left (Y direction) and the right (-Y direction). In other words, the vertical direction or the Z-axis direction includes the upper direction (Z direction) and the lower direction (-Z direction).
Furthermore, in the figure, those with “•” in “○” mean arrows pointing from the back of the paper to the front, and those with “x” in “○” from the table on the paper It shall mean an arrow pointing to the back.

(Example 1)
Next, a 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 fixing device according to a first embodiment of the present invention.
In FIG. 1, an image forming apparatus U includes a digital copying machine A as an image forming apparatus main body having a platen glass (transparent document table) A1 on an upper surface, and an automatic copying machine A detachably mounted on the platen glass A1. A document conveying device B is provided.
The automatic document feeder B sequentially takes out the documents stored in the document feed tray TR1, conveys them to a copy position on the platen glass A1, and discharges the copied documents to a document discharge tray TR2. It is configured.

  The copying machine A includes an image input terminal IIT (hereinafter, IIT) as an image reading unit and an image output terminal IOT (hereinafter, IOT) as an image recording operation unit sequentially arranged below the platen glass A1. have.

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 document illumination mounted on a full-rate carriage and a first mirror 3. 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 half the moving speed of the full-rate carriage. It has an imaging lens 6 supported.
The imaging lens 6 has a function of converging a document reflected light reflected from the document on the platen glass A1 and further reflected by the first, second, and third mirrors to a CCD (solid-state imaging device). The CCD has a function of converting the original reflected light converged on the imaging surface into an electric signal.
The IIT has an image data output unit 7 that receives an electric signal obtained by the CCD and outputs it as digital image data, and a user interface UI that allows a user to input an operation command signal such as copy start. The user interface UI includes a coated paper input button 59 (not shown) as coated paper discriminating means for designating and inputting that the recording sheet S is coated paper, and also relates to a current setting state of the image forming apparatus U. It has a display unit (not shown) for displaying information and the like.

The IOT includes a laser driving circuit 8 that converts image data output from the image data output unit 7 into a laser driving signal, and a laser beam for writing an image in accordance with the laser driving signal output from the laser driving circuit 8. A laser writing device 9 for emitting L is provided.
The laser beam L emitted from the laser writing device 9 is incident on the surface of the image carrier 11 at a latent image writing position Q1 set on the movement path of the surface of the image carrier 11 rotating clockwise in FIG. It is supposed to.
A charger 12 is arranged along the surface of the image carrier 11 on the upstream side in the rotation direction of the latent image writing position Q1.

A developing unit 13 for developing an electrostatic latent image formed on the surface of the image carrier 11 into a toner image is disposed downstream of the latent image writing position Q1 in the rotation direction along the surface of the image carrier 11. Have been. The developing unit 13 has a red developing unit 13a and a black developing unit 13b. A transfer unit 14 is disposed at a transfer position Q2 set further downstream of the developing unit 13. Further, a cleaner unit 16 is disposed further downstream in the rotation direction of the transfer device 14.
Sheet feed trays 21 to 25 are arranged in multiple stages below the transfer device 14. The paper feed tray 23 is an intermediate tray used when circulating a recording sheet (hereinafter referred to as a sheet) S on which a first copy has been performed at the time of double-sided copying or multiple copying, and re-transmitting the recording sheet S to the transfer position Q2.
The sheets S in the respective paper feed trays 21 to 25 are transported to the transfer position Q2 through the first sheet transport path 26. The transfer device 14 is disposed at the transfer position Q2, and is configured to transfer the toner image on the surface of the image carrier 11 to the sheet S passing therethrough.
After the toner remaining on the surface of the image carrier 11 that has passed the transfer position Q2 is recovered by the cleaner unit 16, the surface of the image carrier 11 is uniformly charged again by the charger 12.

At the end of the first sheet conveyance path 26, a registration gate 27 for temporarily stopping the sheet S conveyed to the upstream side of the transfer position Q2 in the sheet conveyance direction and then carrying the sheet S to the transfer position Q2 is provided. A registration roll 28 is provided. The sheet S to which the toner image has been transferred at the transfer position Q2 is conveyed to a fixing position Q3 through a second sheet conveying path 29 connected to the sheet discharge tray TR3.
The heating and pressing element of the fixing device F is disposed at the fixing position Q3, and is configured to fix the unfixed toner image I on the sheet S passing through the fixing position Q3 by heating and pressing. The second sheet transport path 29 is provided with a discharge roller 31 for discharging sheets to the sheet discharge tray TR3 downstream of the fixing position Q3.

In the second sheet transport path 29, a switching gate 32 is disposed upstream of the discharge roller 31. The switching gate 32 is used when switching 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 a sheet reversing 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 reversing path 34 when performing double-sided copying, and directly toward the intermediate tray 23 when performing multiple copying. The sheet-shaped and comb-shaped mylar gate 37 provided in the sheet reversing path 34 allows the sheet S to move downward by elastic deformation when the passing sheet S is conveyed downward. When the sheet S that has passed through the switch 37 is switched back and conveyed upward, the sheet S is guided in the direction of the intermediate tray 23.
The sheet S once stored in the intermediate tray 23 is configured to be transported again from the intermediate tray 23 to the transfer position Q2 by the first sheet transport path 26.

Next, the fixing device F will be described.
FIG. 2 is an explanatory diagram of a fixing device of a belt nip type according to the first embodiment of the present invention. FIG. 2A is a diagram showing a state where a normal recording sheet is fixed, and FIG. 2B is a diagram showing a state where a coated paper is fixed. . 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 is an aluminum cylinder having an outer diameter of 46 mm and an inner diameter of 42 mm. The surface of the core 42 is directly coated with an HTV silicone rubber having a hardness of 45 ° (JIS-A) with a thickness of 2 mm as a base layer 43, and further a Viton rubber (registered trademark of DuPont) as a top coat layer 44 thereon. It is coated with a thickness of 30 μm. The underlayer 43 and the top coat layer 44 constitute an elastic body surface layer (43 + 44). The outer diameter of the heat fixing roll 41 has the following value.
Outer diameter of the 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 controller (not shown) feedback-controls the output of the halogen lamp 46 based on the signal of the temperature sensor 47 to control the surface of the heat fixing roll 41 Is normally adjusted to 160 ° C. during fixing of the recording paper. A release agent (an 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 as a pressing member that is pressed against the heat fixing roll 41 is formed of a polyimide film to have a thickness of 75 μm, a width of 300 mm, and a circumference of 188 mm. The endless belt 51 is wound around the belt supporting rolls 52 and 53 and the pressing roll 54 with a tension of 98.06 N. The belt supporting rolls 52 and 53 and the pressing roll 54 are formed of stainless steel, and their diameters are 18 mm, 18 mm and 23 mm, respectively.

The belt supporting rolls 52 and 53 and the pressing roll 54 are each tapered so that the diameter of the center portion is slightly larger than the diameter of the end portion. Even if the endless belt is bent, the endless belt 51 becomes flat and runs smoothly without waving.
Of these rolls, the pressing roll 54 is pressed toward the center of the heating and fixing roll 41 by a constant pressing force (load) by the compression spring 56, whereby the endless belt 51 is pressed against the heating and fixing roll 41. It is pressed. This pressure contact structure will be described later in detail.

One end (lower end in the figure) of the compression spring 56 is fixed to the copying machine A main body, and the other end (upper end) is fixed to a bearing holder of a rotating shaft (not shown) of the pressing roll 54 to reduce the pressing force. In addition.
In the case of fixing a normal sheet in FIG. 2A, the winding angle (wrap angle) of the endless belt 51 around the heat fixing roll 41 is 45 °. Since the outer diameter of the heat fixing roll 41 is 50.06 mm, the pressure contact area between the heat fixing roll 41 and the endless belt 51 having a wrap angle of 45 ° (that is, the width of the fixing nip N where the fixing is performed) ) Is the next value.
N = 50.06 × 3.14 × 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 heat fixing roll 41, an endless belt pressed toward the elastic surface layer (43 + 44) of the heat fixing roll 41 by the pressing roll 54. Reference numeral 51 denotes a state in which the surface of the heat fixing roll 41 has been cut. In this state, the endless belt 51 does not easily slide with respect to the heat fixing roll 41, and can run at the same speed as the peripheral speed of the heat fixing roll 41.
In this embodiment, the nip pressure P1 of the endless belt 51 pressed by the pressing roller 54 against the heat fixing roller 41 is set to the following value.
P1 = 4 kg / cm ²

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 is configured so that the direction of its axis can be adjusted. Have been. That is, the shaft (the front end in the drawing) of the belt supporting roll 52 is supported so as to be slightly displaceable to the left and right in the drawing as indicated by the arrow in FIG.
An auxiliary pad 57, which is an auxiliary pressing member disposed on the upstream side in the traveling direction of the sheet S with respect to the pressing roll 54, is provided with a 2.5 mm thick silicone sponge 57b (silicone rubber foam) on a 10mm wide stainless steel support 57a. The upper layer is covered with a Teflon (registered trademark) sheet 57c in which glass fibers are impregnated with Teflon (registered trademark) in order 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 heat fixing roll 41 by the auxiliary pad 57 is set to the following value.
P2 = 4 kg / cm ²

Since the heat fixing roll 41 is rotated at a peripheral speed Vo = 250 mm / sec by a drive motor 49 using a step motor or the like, the endless belt 51 pressed against the heat fixing roll 41 also runs at a speed Vo = 250 mm / sec. It is made to be made. The peripheral speed of the heat fixing roll 41 is determined according to the rotation speed of the drive motor 49, and is calculated from the drive pulse of the drive motor 49 generated in the controller C.
As described above, the nip passing time t of the sheet S when N = 19.75 mm and Vo = 250 mm / sec is as follows.
t = N / Vo
= 19.75 / 250 = 79 msec

FIG. 3 is an explanatory diagram of a wrap width variable mechanism of the belt nip type fixing device according to the first embodiment of the present invention.
A detection signal that the sheet S is coated paper is input to a copying machine controller C (hereinafter, controller C) from a coated paper input button 59 of the UI (user interface). The controller C moves the support roll 53 from the position shown in FIG. 2A so as to increase the winding angle (that is, the wrap angle) of the endless belt 51 around the heating fixing roll 41 to increase the width of the fixing nip N. It is adapted to be raised to the position of FIG. 2B.

3, the shaft 52a of the support roll 52 and the pressing roll shaft 54a are held at a fixed distance by an arm 61. The arm 61 is rotatably supported around the support roll shaft 52a, and the rotation shaft 54a of the pressing roll 54 presses the endless belt 51 against the surface of the heat fixing roll 41 by the compression spring 56. It has become.
The wrap width variable mechanism 62 for changing the wrap width (wrapping width) of the endless belt 51 on the surface of the heat fixing roll 41 has an arm 63 supported rotatably about the support roll shaft 52a.
A spring housing groove 63a is cut in a longitudinal direction at a tip of the arm 63, and a compression spring 64 is housed therein. The compression spring 64 is arranged so as to freely press the bearing holder 53b of the rotation shaft 53a of the support roll 53 toward the tip of the arm 63, and the pressing force is balanced with the tension of the endless belt 51. I have. Further, a gear 66 formed along an arc concentric with the support roll 52 is fixed near the support roll 52 on the arm 63, and the arc gear 66 meshes with a small gear 67. The wrap width variable mechanism 62 (63 to 64 + 66 to 67) of the first embodiment is configured by the elements indicated by the reference numerals 63 to 64 and 66 to 67.

The small gear 67 is connected to a step motor 68 for controlling the lap width, and the step motor 68 is connected to the controller C. When the coated paper input button 59 of the UI (user interface) is input, a coated paper detection signal is input to the controller C. When the step motor 68 rotates forward and reverse in response to a drive command from the controller C, the small gear 67 also rotates forward and reverse. As a result, when the small gear 67 rotates, for example, in a clockwise direction (arrow direction in the figure), the lap width variable mechanism 62 rotates the arm 63 in a counterclockwise direction (arrow direction in the figure) to support the arm 63. The roll 53 is raised to increase the wrap angle.
In the first embodiment, the wrap angle is set to 57 °, which is increased from 45 ° for plain paper to 12 ° (that is, about 25%) for coated paper.
That is, the controller C stores in the built-in memory a required lap angle of 57 ° when the sheet S is coated paper and a table of the corresponding signal for driving the step motor 68 in the built-in memory. Upon receiving the signal, a required drive signal is read from the table and transmitted to the step motor 68 via the drive circuit D3 to drive.

The wrap width of the coated paper (that is, the width of the nip N) at the wrap angle of 57 ° is as follows.
N = 50.06 × 3.14 × 57/360
= 25.02 (mm)
The nip passage time t of the sheet S when N = 25.02 mm and Vo = 250 mm / sec is as follows.
t = N / Vo
= 25.02 / 250 = 0.1 sec = 100 (msec)
As described above, the nip passage time control means (nip area increasing means) 69 (62 + 68 + C + T) of the fixing device according to the first embodiment of the present invention is constituted by the elements denoted by reference numerals 62, 68, C and T.

(Operation of First Embodiment)
Next, the operation of the fixing device F according to the first embodiment having the above-described configuration will be described.
In FIG. 1, in response to a copy command signal input from a user interface UI, the automatic document feeder B sequentially takes out documents stored in a document feed tray TR1 and moves the document to a copy position on the platen glass A1. Bring in.
In the IIT, the original is travel-exposed by an exposure lamp 2 for illuminating the original of the exposure scanning optical system 1 disposed immediately below the platen glass A1, and the reflected light is reflected by a first mirror 3 and a second mirror 4. Then, the light is reflected by the third mirror 5 and converged on a CCD (solid-state imaging device) through an imaging lens 6. The CCD converts the document reflected light converged on the imaging surface into an electric signal and outputs the signal, and the image data output unit 7 receiving the converted signal converts the signal into digital document image data and outputs the digital document image data. The automatic document feeder B takes out the copied document from the copy position and discharges it to the document discharge tray TR2.

In the IOT, the laser drive circuit 8 receiving the image data converts the image data into a laser drive signal, and sends the laser drive signal to the laser writing device 9. The laser writing device 9 irradiates the laser beam L for image writing to the latent image writing position Q1 on the surface of the image carrier 11 in response to this signal. Although the surface of the image carrier 11 is uniformly charged in advance by the charger 12, an electrostatic latent image corresponding to a document 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 attaches toner to the electrostatic latent image and develops the electrostatic latent image into a toner image. At that time, the red developing unit 13a or the black developing unit 13b is switched and used as needed. At the transfer position Q2 set further downstream, the transfer unit 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 further scraped off by the cleaner unit 16 on the downstream side in the rotation direction.

The sheet S to which the toner image has been 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 completed, the sheet is conveyed downstream through the second sheet conveying path 29. 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 a corresponding tray (excluding the paper feed tray 23) among the paper feed trays 21 to 25, and passes through the first sheet transport path 26. The sheet is conveyed until it comes into contact with the registration gate 27 at the end. The registration gate 27 passes the stopped sheet S in synchronization with the arrival of the toner image on the image carrier 11, and sends the sheet S to the transfer position Q2 by the registration roll.

  In cases other than the single-sided single copy, the switching gate 32 provided in the second sheet conveying path 29 is switched to convey the single-sided transferred sheet S to the lower sheet circulation path 33. Then, the lower switching gate 36 is operated to convey the sheet S directly to the intermediate tray 23 in the case of multiple copying. In the case of double-sided copying, the sheet S is once fed into the lower sheet reversing path 34 to be switched back, and the sheet S turned upside down by the mylar gate 37 is sent to the intermediate tray 23. After temporarily stopping the sheet S at a predetermined position on the intermediate tray 23, the sheet S is transported again to the transfer position Q2 through the first sheet transport path 26.

Next, the operation of the fixing device F will be described with reference to FIGS.
First, when the sheet S conveyed from the right side of FIG. 2 to the fixing position Q3 is not coated paper (in the case of a normal recording paper), the sheet S is transferred onto the endless belt 51 of FIG. 49, the fixing nip N having a wrap width (= 19.75 mm) determined by the wrap angle 45 ° which is a pressure contact area between the heating fixing roll 41 and the endless belt 51 rotating and rotating at a peripheral speed of Vo = 250 mm / sec. Sent to. Therefore, during the time t = 79 msec for passing through the nip N (= 19.75 mm), the heat of the heat fixing roll 41 at the surface temperature of 160 ° C. and the clamping force between the heat fixing roll 41 and the endless belt 51 are used. Thus, the unfixed toner I is fixed on the sheet S.

That is, first, the heat fixing roll 41 is heated by the halogen lamp 46 disposed at the center, and the surface thereof is uniformly heated. The surface temperature is detected by the temperature sensor 47, and is maintained at 160 ° C. under feedback control via the controller C. On the surface of the heat fixing roll 41, a release agent of silicone oil is supplied from the oil supply device 48 (FIG. 1) on the upstream side to form a thin film, which is formed by the heat fixing roll of the unfixed toner I. The transfer to 41 is suppressed.
The pressure required for the heat fixing is a steady pressure caused by the endless belt 51 being pressed at a winding angle (wrap 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 a pressing force for pressing the inner surface of the endless belt 51.

Next, a case where the fed sheet S is a coated paper will be described with reference to FIGS. When the coated paper input button 59 of the UI (user interface) is input, a detection signal indicating that the sheet S carried into the fixing nip N is coated paper is sent to the controller C. Upon receiving this input, the controller C reads a drive signal of the step motor 68 corresponding to a required lap width when the sheet S is coated paper from a table stored in the internal memory, The signal is transmitted to the step motor 68 via the circuit.
This signal causes the step motor 68 to rotate by a required angle, and the small gear 67 of the wrap width variable mechanism 62 (63 to 64 + 66 to 67) coupled thereto to rotate clockwise as shown in FIG. When the arm 63 is rotated in the direction indicated by the arrow, the arm 63 is rotated in the counterclockwise direction (the direction indicated by the arrow in the figure), and the support roll 53 is raised from the position corresponding to the normal recording paper indicated by the broken line to the position indicated by the solid line. , The wrap angle is increased.

That is, the right end point R1 of the pressure contact area of the endless belt 51 with the heat fixing roll 41 on the normal recording paper moves to R2 as the support roll 53 rises, and the width of the fixing nip N is changed from R0 to R1 to R0. To R2. Therefore, when the endless belt 51 is rotating at the same speed as that of the normal recording paper, the passage time of the fixing nip N increases by (R0 to R2) / (R0 to R1) times.
As described above, the value of the fixing nip N and the nip passing time t are as follows.
(In the case of plain paper)
The fixing nip N of the above (R0 to R1) = 19.75 mm
The fixing nip passing time t (79 msec) of the sheet S in the above (R0 to R1)
(In the case of coated paper)
The fixing nip N of (R0 to R2) = 25.02 mm
The fixing nip passing time t of the sheet S (R0 to R2) is 100 msec.

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

  The relationship between the increase in the width of the fixing nip N and the prevention of blisters in the first embodiment will be described based on the knowledge obtained from the experimental results (FIGS. 4 and 5) of the inventor. In the experiment, the generation of blisters (paper blisters, toner blisters) by changing the fixing roll temperature (that is, the surface temperature of the heated 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) were changed. The change in the fixable temperature was examined. As the sheet S, 90 gsm and 163 gsm double-sided coated paper was used. FIG. 4 is a diagram showing the relationship between blister generation, fixing temperature, nip pressure, and heating time in the case of 90 gsm double-side coated paper. FIG. 5 is a graph showing the relationship between blister generation, fixing temperature, nip pressure, and heating time in the case of 163 gsm double-side coated paper.

FIG. 4A shows a case where the nip pressure by the auxiliary pad 57 is 6 kg / cm ² , the nip pressure is high, and blisters are easily generated.
In FIG. 4A, the broken line at the upper right of the line indicates the limit of paper blister generation, and indicates that paper blister is generated at a higher fixing temperature. The second polygonal line similarly indicates the limit of toner blister occurrence. Similarly, the third (lowermost) broken line indicates that a fixing failure occurs at the lowest fixing temperature, that is, at a fixing roll temperature lower than this. Therefore, the area between the second polygonal line and the lowermost polygonal line is an appropriate area for fixing and preventing blisters. This area is called a usable area in the figure. The above definitions are the same in FIGS.
In FIG. 4A, when the fixing roll temperature of the coated paper is set to 160 ° C. which is the same as that of plain paper, it is difficult to perform fixing without generating blisters. However, when the fixing roll temperature of the coated paper is 130 ° 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 becomes small. Expand up and down. Therefore, it is easy to always maintain the fixing temperature and the fixing time within the usable area even if the fixing roll temperature slightly changes.

As shown in FIGS. 4 and 5, when the nip pressure is as small as about 2 kg / cm ² , it is relatively easy to avoid the occurrence of toner blisters even when the fixing time is 80 msec or less. However, when the nip pressure is relatively large, about 6 kg / cm ^2, it is difficult to avoid the occurrence of toner blisters when the fixing time is 80 msec or less. In general, when the nip pressure is small, toner blisters are hardly generated even if the fixing time is short, but as the nip pressure is increased, when the fixing time is short, toner blisters are easily generated.
Regardless of the nip pressure, the generation of toner blisters can be reduced by setting the fixing time to 80 msec or longer.

In the first embodiment, the variable mechanism of the wrap angle is operated with respect to the surface peripheral speed of the heat fixing roll 41 of 250 mm / sec and the width of the fixing nip N of 19.6 mm in the case of plain paper (lap angle of 45 degrees). If the width of the fixing nip N is increased by about 25%, for example, the heating time becomes 100 msec (wrap 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 to 5B, appropriate positions (combinations of fixing conditions) in the usable area differ from each other.

(Example 2)
FIG. 6 is a schematic configuration diagram of a roll-pair fixing device (when a large-diameter roll pair and a small-diameter roll pair are used together) in Embodiment 2 of the present invention.
In the description of the second embodiment, components corresponding to the components of the first embodiment are denoted by the same reference numerals, and a 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 a pressure contact member in contact with the surface of the heat fixing roll 41, and the belt supporting rolls 52 and 53 and the pressing roll 54, the heat fixing roll 41 as the pressure contact member A pressure roll 71 having the same diameter as that of the heat fixing roll 41 is disposed from below and pressed against the surface of the heat fixing roll 41. The materials of the core 72, the underlayer 73, and the topcoat layer 74 in the components of the pressure roll 71 are the same as the core 42, the underlayer 43, and the topcoat layer 44 of the heat fixing roll 41, respectively. Is formed thinner than the underlayer 43. Thus, the pressure roll 71 elastically deforms the heat fixing roll 41 to form the fixing nip N by slightly depressing the surface, so that self-stripping can be performed.

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

A sheet carry-in device 78 for carrying the sheet S into the fixing nip N is disposed on the right side (−Y direction) of the heat fixing roll 41. The sheet carry-in device 78 includes a belt drive roll 79 and a belt support roll 81, and a sheet transport belt 82 stretched around them. The belt drive roll 79 and the belt support roll 81 rotate counterclockwise around the rotation shafts 79a and 81a, respectively, so as to carry the sheet S on the sheet transport belt 82 toward the fixing nip N. ing. The rotating shaft 79a is configured to be rotationally driven by a driving device (not shown).
A large-diameter roll fixing unit W is configured by a combination of the heat fixing roll 41, the pressure roll 71, and the sheet carrying-in device 78.

In the second embodiment, another relatively small-diameter roll fixing unit W ', which is configured similarly to the above-described large-diameter roll fixing unit W, is disposed above. Elements indicated by reference numerals 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 carry-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.
Accordingly, 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 section W' is also smaller than the width of the fixing nip N in the large-diameter roll fixing section W. Has become. Also, the width of the fixing nip N 'can be adjusted similarly to 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 'are as follows, and the fixing temperatures T and T' are the same.
N = 8mm
Nip pressure P = 6 kg / cm ²
Fixing temperature T = 130 ° C
N '= 4mm
Nip pressure P '= 6 kg / cm ²
Fixing temperature T '= 160 ° C

On the upstream side of the belt support rolls 81 and 81 ′ of the large-diameter roll fixing unit W and the small-diameter roll fixing unit W ′, a belt-type movable sheet conveying device 83 is disposed, and the downstream of the sheet conveying device 83 is disposed. The side belt support roll 84 is supported by an arm 85, and is configured to be vertically adjustable together with the downstream end of the arm 85 by a solenoid 86. When the belt support roll 84 is moved upward (the solenoid 86 is turned off), the sheet conveyed by the sheet conveyance device 83 is conveyed to the small-diameter roll fixing unit W ′, and the belt support roll 84 is moved downward. The sheet S is conveyed to the large-diameter roll fixing unit W in a state where the sheet S is moved to the state (the solenoid 86 is turned on).
In the second embodiment, the sheet carry-in devices 78 and 78 'each constitute a part of the second transport path 29.

A coated paper detection signal from a coated paper input button 59 of the UI (user interface) is input to the controller C. Upon receiving this, the solenoid 86 is energized by a drive signal from the controller C. The on / off operation of the solenoid 86 adjusts the vertical position of the belt supporting roll 84 of the belt-type movable sheet conveying device 83.
As described above, the nip passage time control means (nip area increasing means) 69 (59 + 83 to 86 + C + W + W ') of the fixing device according to the second embodiment of the present invention is constituted by the elements 59, 83 to 86, W, W', and C. ing.

(Operation of the Second Embodiment)
Next, the operation of the fixing device having the above-described configuration according to the second embodiment of the present invention will be described.
The operation of the second embodiment is different from that of the first embodiment in the following points, but is otherwise the same as that of the first embodiment.
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 S is passed through the upper sheet conveying belt 82 'as shown in FIG. The heating fixing roll 41 ′ which is fed into the fixing nip N ′ of the small-diameter roll fixing section W ′ and is rotated at a peripheral speed of 200 mm / sec by the drive motor 49 ′, and the pressure roll 71 which is pressed against the heating fixing roll 41 ′ ′, The unfixed toner I is fixed on the sheet S by heating and pressing. The sheet S that has been fixed is sent to the left downstream (not shown).
In this case, the nip passing time, that is, the fixing time t 'and the fixing temperature T' are as follows.
t '= 4 mm / 200 mm / sec
= 20msec
T '= 160 ° C

  Next, the case of coated paper will be described. When a coated paper detection signal is input from the coated paper input button 59 of the UI (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 a drive circuit D4. The energized solenoid 86 moves the belt support roll 84 downward.

Due to the lowering of the belt supporting roll 84, the sheet S from the movable sheet conveying device 83 is sent to the fixing nip N of the large-diameter roll fixing unit W through the lower sheet conveying belt 82, Heating and pressure are applied and fixed by the heat fixing roll 41 and the pressure roll 71 at a peripheral speed of 100 mm / sec. In this case, the nip passing time, that is, the fixing time t and the fixing temperature T are as follows.
t = 8mm / 100mm / 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, fixing is performed at a lower fixing temperature of 130 ° C. and a longer fixing time of 80 msec than in the small-diameter roll fixing section W ′.
When the coated paper detection signal from the coated paper input button 59 is released, the solenoid 86 is turned off, the belt support roll 84 rises, and the sheet S from the movable sheet conveying device 83 is released. Is fed into the fixing nip N 'of the small-diameter roll fixing section W' via the upper sheet conveying belt 82 ', and is heated for a short time corresponding to plain paper.
The above is the operation of the nip passage time control means (nip area increasing means) 69 (59 + 83 to 86 + C + W + W ') of the second embodiment.

(Example 3)
Next, a third embodiment of the present invention will be described with reference to FIGS. 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 the fixing speed control of the fixing device in the third embodiment.
In the third embodiment, the same reference numerals are given to the components corresponding to the components of the first embodiment, and the detailed description is omitted.
The third embodiment differs from the first embodiment in the following points, but has the same configuration as 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 against the heat fixing roll 41, and the wrap width variable mechanism 62, A roll 71 is provided. The nip passage time control is performed by nip passage speed control for controlling the rotation speed of the drive motor 49 of the heat fixing roll 41.

Depending on whether the sheet S is plain paper or coated paper, the control parameters are set as follows. The width of the nip pressure P and the width of the nip N do not change for plain paper and coated paper.
(In the case of plain paper)
Nip pressure P = 6 kg / cm ²
N = 8mm
Circumferential velocity Vo of heating fixing roll 41 = 250 mm / sec
Fixing temperature T = 160 ° C
(In the case of coated paper)
Nip pressure P = 6 kg / cm ²
N = 8mm
Circumferential velocity Vo of heating fixing roll 41 = 100 mm / sec
Fixing temperature T = 130 ° C

When the parameters are set as described above, the nip passage time t is as follows.
(In the case of plain paper)
t = (8 mm) / (250 mm / sec)
= 32 msec
(In the case of 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. The flowchart of FIG. 8 includes a discriminating step ST1. In the case of a normal recording sheet, steps ST2 and ST3 for fixing at a normal fixing roll temperature and a fixing speed are operated. Steps ST4 and ST5 for fixing at a low fixing speed are operated.

(Operation of Embodiment 3)
Next, the operation of the fixing device according to the third embodiment of the present invention having the above-described configuration will be described with reference to FIGS.
In step ST1 of FIG. 8, it is determined whether the fixing paper is a coated paper. This determination is made based on whether or not the coated paper input button 59 of the UI (user interface) has been input. In the case of No (N), since the paper is plain paper, in step ST2, the halogen lamp 46 and the driving motor are set so that the fixing temperature T is 160 ° C. and the peripheral speed Vo of the heat fixing roll 41 is 250 mm / sec. Activate 49.

When the coated paper input button 59 of the UI (user interface) is input, the answer is yes (Y) in step ST1. In this case, the process moves to step ST3.
In step ST3, the control value of the heating temperature (the surface temperature of the fixing roll) is set to 130 ° C. Next, the process moves to step ST4.
In step ST4, the set value of the fixing speed (the peripheral speed of the heat fixing roll 41) Vo is set to 100 mm / sec.
Thus, the fixing process is performed on the coated paper in an appropriate fixing condition area.
The above is the operation of the fixing temperature control means 47 + 59 + C and the nip passage time control means (nip passage speed control means) 49 + 59 + C in the third embodiment.

(Example 4)
FIG. 9 is an overall explanatory diagram of a fixing device according to a fourth embodiment of the present invention. Note that the fixing speed control of the fixing device in the fourth embodiment performs the same processing as the flowchart shown in FIG. 8 of the third embodiment.
In the fourth embodiment, the same reference numerals are given to the components corresponding to the components of the first embodiment, and the detailed description is omitted.
The fourth embodiment differs from the first embodiment in the following points, but has the same configuration as the first embodiment in other points.
That is, in the fixing device F of the fourth embodiment, the variable wrap width 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, so that the rotation speed of the heat fixing roll 41 is reduced 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 heat 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.
(In the case of plain paper)
Nip pressure P = 6 kg / cm ²
N = 19.75mm
Circumferential velocity Vo of heating fixing roll 41 = 250 mm / sec
Fixing temperature T = 160 ° C
(In the case of coated paper)
Nip pressure P = 6 kg / cm ²
N = 19.75mm
Circumferential velocity 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.
(In the case of plain paper)
t = (19.75 mm) / (250 mm / sec)
= 79 msec
(In the case of coated paper)
t = (19.75 mm) / (200 mm / sec)
= 98.75msec

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 includes a discrimination step ST1. In the case of ordinary 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 are activated. Steps ST4 and ST5 for fixing at a roll temperature of 130 ° C. and a low fixing speed of 200 mm / sec operate.
Therefore, the fourth embodiment has the same operation as the third embodiment.

(Example 5)
Next, a fifth embodiment of the present invention will be described with reference to FIG. FIG. 10 is a schematic diagram of a preheating apparatus according to Embodiment 5 of the present invention.
In the description of the fifth embodiment, components corresponding to the components of the first embodiment are denoted by the same reference numerals, and a detailed description thereof will be omitted.
The fifth embodiment is different from the first embodiment in the following points, but has the same configuration as the first embodiment in other points.
In the fifth embodiment, the variable wrap width mechanism of the first embodiment is omitted. Further, a preheating device 87 is disposed upstream of the fixing device F and above the second transport path 29.

The preheating device 87 includes a housing 88 that covers the upper part, and a heater 89 that is attached immediately below the housing 88 at a certain interval. A plurality of heaters 89 are provided orthogonally to the transport direction of the second transport path 29, and are turned on / off at a predetermined input power by a signal from the controller C. Thereby, when the sheet S conveyed on the second conveyance path 29 is coated paper, the sheet S is preheated. The lower surface of the housing 88 is covered with a reflective metal surface, and is formed in a shape such that the heat of the heater 89 can be efficiently reflected and the surface of the sheet S can be preheated uniformly.
Further, the controller C has a simple program that replaces the program and the table of the second embodiment. The simple program is necessary for preheating the sheet S conveyed to the preheating device 87 to 60 ° C. or higher when a coated paper detection signal is received from the coated paper input button 59. The data of the operation timing, operation time, and heater input power of the heater 89 are stored. When receiving the coated paper detection signal, the simple program extracts the data, converts the data into an operation signal of the heater 89, and transmits the signal to the preheating device 87.
The preheating means (59 + 87 + C) of the fixing device according to the fifth embodiment of the present invention is constituted by the elements denoted by reference numerals 59, 87 and C.

(Operation of Embodiment 5)
Next, the operation of Embodiment 5 of the fixing device of the present invention having the above-described configuration will be described.
The operation of the fifth embodiment is different from that of the second embodiment in the following points, but is otherwise the same as that of the second embodiment.
When a coated paper input button 59 of the UI (user interface) is input and a coated paper detection signal is sent to the controller C, the controller C turns on the heater 89 and sets a predetermined heating temperature to a predetermined value. The sheet S conveyed on the second conveying path 29 after being heated for a time is preheated to 60 ° C. or higher.
If 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.
Thus, pre-fixing release of air and moisture in the coated paper by preheating is performed.

  In addition, the knowledge obtained from the inventor's experimental results regarding the relationship of blister prevention by the preheating in Example 5 is as shown in FIG. That is, the preheating temperature before fixing was changed between 20 ° C. and 90 ° C. for the two types of coated paper having the coating amounts of 90 gsm and 163 gsm, and the occurrence of toner blisters was observed. It has been found that when the above preheating is performed, generation of toner blister is prevented. Based on the above findings, the preheating temperature in Example 5 was set to 60 ° C. or higher.

(Example 6)
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. FIG. 13 is a detailed explanatory view of the nip pressing force control means according to the sixth embodiment of the present invention. FIG. 13A shows the configuration of the pressure roll supporting device, and FIG. 13B shows the configuration of the pad supporting device.
In the description of the fifth embodiment, components corresponding to the components of the first embodiment are denoted by the same reference numerals, and a detailed description thereof will be omitted.

In FIG. 12, the pressing roll 54 is pressed toward the center of the heating and fixing roll 41 with the pressing force adjusted by the pressing roll supporting device 91, whereby the endless belt 51 is pressed against the heating and fixing roll 41. ing. 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 a pad support device 92.
In FIG. 13A, the outside of the pressing roll support device 91 is constituted by 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 cylinder surface of the cylinder 93. It has become. Inside the cylinder 93, a compression spring 96 is housed above and a spring receiving piston 97 is housed below. A slider 101 is slidably supported above the cylinder 93. The slider 101 is connected to a bearing case 102 that rotatably supports a rotating shaft 54a of the pressing roll 54 by using a connecting bolt 103.

A piston rod 98 that 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 is driven to rotate around an eccentric shaft 104a by a step motor 106. The step motor 106 is driven by a drive circuit 107 connected to the controller C as shown in FIG. The pressing roll support device 91 is composed of the elements indicated by the reference numerals 93 to 103. The pressing roll supporting devices 91 are disposed at both front and rear (X-direction) ends in the copying machine A.

The pad supporting device 92 for the auxiliary pad 57 in FIG. 13B is also configured and arranged similarly to the pressing roll supporting device 91 for the pressing roll 54. That is, the pad support device 92 is also constituted by elements 93 'to 103' corresponding to the elements indicated by reference numerals 93 to 103 shown in FIG. 13A. Among the elements 93 'to 103', only the pad connecting member 102 'is different from the bearing case 102, but the other elements 93' to 101 'and 103' have the same configuration as the elements 93 to 101 and 103. Have been.
Further, a step motor 106 'for driving 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 accommodated in the cylinder 93 so as to be slidable and rotatable at the same time. However, since the sliders 101 are arranged in a pair in the front-rear direction (X direction) and connected by the pressure roll shaft 54 extending in the front-rear direction, the sliders 101 do not rotate in the cylinder 93. In order to reliably prevent the rotation of the slider 101 in the cylinder 93, a guide groove extending in the axial direction and a plurality of slidable and non-rotatably engaged grooves in the cylinder 93 and the slider 101 are provided. Guide pins may be provided.

In FIG. 12, a coated paper detection signal from a 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 for generating appropriate pressing force values (a nip area pressure of 2 kg / cm ² or less, which will be described later) of the pressing roll supporting device 91 and the pad supporting device 92 corresponding to various (gsm) coated papers. It has a memory that stores a table of the rotational position data of the step motors 106 and 106 '. By rotating the step motors 106 and 106 'until the rotation position read from the table is reached, a predetermined pressing force can be generated.

The roll pressing force adjusting device J is constituted by the elements indicated by the aforementioned reference numerals 59, 104, 106, 107 and C. Similarly, a pad pressing force adjusting device J 'is constituted by elements indicated by reference numerals 59, 104', 106 ', 107' and C. The elements denoted by reference numerals 91, 92, J and J 'constitute a nip pressing force control means K (91 + 92 + J + J') according to the sixth embodiment of the fixing device of the present invention.
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 for plain paper and coated paper.
(In the case of plain paper)
N = 19.75mm
Nip pressure P1 of pressing roll 54 = 4 kg / cm ²
Nip pressure P2 of auxiliary pad 57 = 3 kg / cm ²
Circumferential velocity Vo of heating fixing roll 41 = 250 mm / sec
Fixing temperature T = 160 ° C
(In the case of coated paper)
N = 19.75mm
Nip pressure P1 of pressing roll 54 = 4 kg / cm ²
Nip pressure P2 of auxiliary pad 57 = 2 kg / cm ²
Circumferential velocity 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.
(In the case of plain paper)
t = (19.75 mm) / (250 mm / sec)
= 79 msec
(In the case of coated paper)
t = (19.75 mm) / (200 mm / sec)
= 98.75msec

  The nip pressing force control means K (91 + 92 + J + J ') of the sixth embodiment is configured to be able to adjust the pressing force on both the pressing roll 54 side and the auxiliary pad 57 side. It can be configured to adjust the pressing force.

(Operation of Embodiment 6)
Next, the operation of Embodiment 6 of the fixing device of the present invention having the above-described configuration will be described.
In FIG. 12, when the fixing paper is plain paper, the rotational position of the step motors 106, 106 'is determined by the pressed plates 99, 99 on which the eccentric cams 104, 104' are mounted. ′ Are raised so that the nip pressure P1 of the pressing roll 54 is 4 kg / cm ² and the nip pressure P2 of the auxiliary pad 57 is 3 kg / cm ² .

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

As a result, the pressing force against the pressing roll 54 and the auxiliary pad 57 decreases, and the nip pressure P1 of the pressing roll 54 is maintained at 4 kg / cm ² , and the nip pressure P2 of the auxiliary pad 57 is maintained at ² kg / cm ^2. .
This makes it possible to occupy a position of an appropriate fixing condition within a usable area where the occurrence of blisters in the conveyed 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 released, the appropriate one of the step motors 106 and 106 'corresponding to the general recording paper is obtained from the table of the processing program. The rotational force 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 described above.
As can be seen from FIGS. 4 and 5, by reducing the nip pressure, blisters are less likely to occur. Therefore, in the sixth embodiment, in which the fixing time is extended and the nip pressure is reduced when the fixing paper is coated paper, the occurrence of blisters on the coated paper can be reduced.
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 the coated paper, and the set pressures of the nip pressures P1 and P2 are changed according to the coating amount. It is possible to put.

(Example of change)
As mentioned above, although the Example of this invention was described in full detail, this invention is not limited to the said Example, Various changes are made within the range of the gist of this invention described in the claim. It is possible. Modified embodiments of the present invention will be exemplified below.
(H01) Instead of the non-contact type preheating means shown in the fifth embodiment, a heating roll or a heating plate which contacts and heats from the back side (inside) of the endless belt for conveying the recording sheet, or uses a heating method for the recording sheet It is also possible to use a heating transport roll (apparatus in which a heater is incorporated in the transport roll) for heating by directly contacting 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 on the user interface shown in the first embodiment. It is.
(H05) The driving type of the fixing device has been described as a type in which the heating fixing roll is driven by a motor throughout all the embodiments. However, instead of this, a type in which a pressure roll below the heating fixing roll is driven (roll pair type, belt-nip type) (In the case of a mold) or a type in which either of the endless belt supporting rolls is driven by a motor (in the case of a belt-nip type or a belt-belt type).
(H06) The oil supply device 48 can be replaced with another release agent application device or can be omitted.

FIG. 1 is a configuration diagram of an entire image processing apparatus including a fixing device according to a first embodiment of the present invention. FIG. 2 is an explanatory diagram of a fixing device of a belt nip type according to the first embodiment of the present invention. FIG. 2A is a diagram showing a state where a normal recording sheet is fixed, and FIG. 2B is a diagram showing a state where a coated paper is fixed. . FIG. 3 is an explanatory diagram of a wrap width variable 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 among blister generation of a coated paper, fixing temperature, nip pressure, and heating time (in the case of a 90 gsm double-sided coated paper). FIG. 5 is a diagram showing the relationship between the occurrence of blisters and the fixing temperature, the nip pressure, and the heating time of the coated paper (163 gsm double coated paper). FIG. 6 is an explanatory view of a roll-pair type fixing device (when a large-diameter roll pair and a small-diameter roll pair are used together) in Embodiment 2 of the present invention. FIG. 7 is an overall explanatory diagram of the fixing device according to the third embodiment of the present invention. FIG. 8 is a flowchart illustrating 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 apparatus according to Embodiment 5 of the present invention. FIG. 11 is a diagram for explaining the operation of the fifth embodiment, showing the relationship between the occurrence of blisters on the coated paper and the preheating temperature. FIG. 12 is an overall configuration diagram of the nip pressing force control means according to the sixth embodiment of the present invention. FIG. 13 is a detailed explanatory view of the nip pressing force control means according to the sixth embodiment of the present invention. FIG. 13A shows the configuration of the pressure roll support device, and FIG. 13B shows the configuration of the pad support device. 14A and 14B are explanatory diagrams of a recording sheet, FIG. 14A is a cross-sectional view of a normal recording sheet (plain paper), and FIG. 14B is a cross-sectional view of a 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 reference numerals

C: controller, F: fixing device, I: unfixed toner image, M: coating layer, N: fixing nip, S: recording sheet (sheet),
29: conveyance path (second sheet conveyance path), 41: heat fixing member (heat fixing roll), 47: temperature sensor, 49: drive motor, 51: endless belt, 54: pressure contact member (pressure roll), 57: pressure contact Member (pressure assist pad), 59: coated paper discriminating means (coated paper input button), 71: pressure roll K (91 + 92 + J + J '): nip pressing force control means,
(49 + 59 + C; 47 + 49 + 59 + C) ... nip passing time control means (nip passing speed control means),
(59 + 87 + C): preliminary heating means 69 (59 + 62 + 68 + C + T; 59 + 84 + 86 + C + W + W '): nip passage time control means (nip area increasing means)

Claims (5)

A fixing device having the following requirements,
(Y01) a heat-fixing member disposed on the recording sheet conveyance path, and a pressing member pressing against the heat-fixing member;
(Y02) the heat-fixing member and the pressure-contact member, which form a fixing nip by a pressure-contact area of the heat-fixing member and the pressure-contact member, and fix an unfixed toner image adhered to the recording sheet passing through the fixing nip;
(Y03) 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 the surface;
(Y04) Nip passing speed control means for controlling the speed of the recording sheet passing through the fixing nip to a lower value than that of plain paper when the coated paper discriminating means judges that the paper is coated paper. The fixing device according to claim 1, wherein the fixing device has the following requirements.
(Y05) Fixing temperature control means for controlling the fixing temperature of the recording sheet (S) to a temperature lower than that of plain paper when the coated paper discriminating means judges that the paper is coated paper. The fixing device according to claim 2, wherein the fixing device has the following requirements.
(Y06) When the coated paper discriminating means determines that the coated paper is coated paper, the pressing force at which the pressing member presses the heating fixing member at the fixing nip is controlled to a lower value than that of plain paper. Nip pressing force control means, A fixing device having the following requirements,
(Y01) a heat-fixing member disposed on the recording sheet conveyance path, and a pressing member pressing against the heat-fixing member;
(Y02) the heat-fixing member and the pressure-contact member, which form a fixing nip by a pressure-contact area of the heat-fixing member and the pressure-contact member, and fix an unfixed toner image adhered to the recording sheet passing through the fixing nip;
(Y03) 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 the surface;
(Y07) Nip passage time control means for making the fixing nip passage time of the recording sheet passing through the fixing nip longer than that of plain paper when the coated paper discriminating means judges that the paper is coated paper. The fixing device according to claim 4, wherein the fixing device has the following requirements.
(Y05) Fixing temperature control means for controlling the fixing temperature of the recording sheet (S) to a temperature lower than that of plain paper when the coated paper discriminating means judges that the paper is coated paper.

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