Fixing device capable of correcting fixing conditions in accordance with an integrated energizing time

Abstract

A fixing device is provided with first and second fixing members each having an elastic member, a heater for heating at least one of the first and second fixing members and a controller. The first and second fixing members are abutted against each other so that the elastic members contact with each other. A sheet carrying an image of a developer is passed between the first and second fixing members, whereby the developer is fixed onto the sheet. The controller corrects fixing conditions including a fixing speed and a fixing temperature in accordance with an integrated energizing time that is an integrated value of an energizing time of the heater.

Description

This application is based on applications Nos. 11-19746 11-19761 and 11-19779 filed in Japan, the contents in which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a fixing device which is used in an image forming apparatus according to the electrophotographic system such as a copying machine, a printer, or a facsimile apparatus to heat and fuse a developer image that has been transferred from an image bearing body to a transfer sheet such as normal paper or a transparency resin sheet for an overhead projector (OHP sheet), thereby fixing the image to the sheet.

Conventionally, as shown in FIG. 46 , a known fixing device of this kind comprises fixing members 3 and 4 configured by a pair of rollers in each of which an elastic member 2 is disposed on the outer periphery of a metal cylinder 1 , heating means 5 for heating the fixing members 3 and 4 , driving means 6 for rotatingly driving at least one fixing member 3 , and a spring 7 which causes the fixing members 3 and 4 to contact with each other at a constant abutting force. A transfer sheet 9 is transported to a contact portion (nip portion 8 ) between the elastic members 2 of the pair of fixing members 3 and 4 . A developer on the transfer sheet 9 is fused by heat supplied from the fixing members 3 and 4 and then fixed to the transfer sheet 9 .

In order to improve the productivity in a copying machine, a printer or the like, the rotational speeds of the fixing members 3 and 4 should be increased so as to increase a speed at which the transfer sheet 9 is passed through the nip portion 8 (fixing speed).

In the conventional fixing device, silicone rubber constituting the elastic members 2 has a heat conductivity of about 0.6 to 0.8×10 −3 cal/cm·sec·° C. Therefore, the efficiency of absorbing heat from the surfaces of the elastic members 2 into the transfer sheet 9 passed through the nip portion 8 is higher than that of supplying heat from the heating means 5 to the surfaces of the elastic members 2 . As a result, a phenomenon takes place in which, when the fixing speed is set to be higher, the surface temperatures of the elastic members 2 are temporarily decreased during a period when an operation of continuously supplying sheets is started. The decrease of surface temperature causes a failure in image gloss or a fixing failure.

In order to increase the efficiency of the heat supply from the heating means 5 to the surfaces of the elastic members 2 , a countermeasure may be taken in which a reinforcing inorganic filler such as silica or iron oxide added to the silicone rubber is replaced with a high thermal conductive filler such as alumina or magnesium oxide, and the addition amount of the filler is increased. However, a high thermal conductive filler has no effect of improving the heat resistance. Furthermore, as the addition amount of the filler is increased, the polymer component having excellent heat resistance is inevitably reduced. Therefore, the elastic members are decreased in strength in high temperature and hence susceptible to be thermally degraded. In the case where the contact abutting force between the fixing members 3 and 4 is constant as in the fixing device shown in FIG. 46 , when the hardness is reduced as a result of thermal degradation, the width of the nip portion 8 (nip width) is increased. When the nip width is increased, the quantity of heat supplied from the nip portion 8 to the transfer sheet 9 is varied and hence the image quality is changed.

Further, when the addition amount of the filler is increased, the hardness of the silicone rubber is increased and the elastic members 2 are decreased in elasticity. When the elasticity of the elastic members 2 is decreased, it is necessary to perform the abutting of the fixing members 3 and 4 at a very high pressure or to thicken the elastic members 2 in order to ensure the width of the nip portion 8 , thereby causing an image to be obscured or a sheet to become crinkled, or reducing the heat supply quantity to cause a fixing failure or the like.

Furthermore, when the density of cross-link of the silicone rubber is increased higher by changing the polymer component, it is possible to suppress the reduction of the hardness due to the thermal degradation. However, the increased density of cross-link causes the elasticity of the elastic members 2 to be decreased.

SUMMARY OF THE INVENTION

As described above, it is difficult to enhance all the heat conductivity, the heat resistance, and the elasticity of silicone rubber constituting the elastic members 2 . When the heat conductivity is set to be high in order to improve the productivity, it is impossible to eliminate an effect of reduction of the hardness due to the thermal degradation. In other words, in order to maintain the image quality, fixing conditions such as the surface temperatures of the elastic members 2 (fixing temperature), and the transporting speed of a transfer sheet (fixing speed) must be changed in accordance with the degree of advancement of thermal degradation.

It is an object of the invention to provide a fixing device in which an effect of reduction of the hardness of an elastic member due to thermal degradation is corrected, thereby attaining a high productivity.

A first aspect of the invention provides a fixing device comprising: first and second fixing members each of which has an elastic member and which are abutted against each other so that said elastic members contact with each other; a heater which heats at least one of said first and second fixing members; and a controller which corrects fixing conditions in accordance with an integrated energizing time that is an integrated value of an energizing time of said heater, wherein a sheet carrying an image of a developer is passed between said first and second fixing members, whereby the developer is heated and fused under a pressure exerted by said first and second fixing members, to be fixed onto the sheet.

In a fixing device of the first aspect of the invention, fixing conditions are corrected in accordance with the integrated energizing time that is the integrated value of the energizing time of the heater. Therefore, even when the efficiency of the heat supply to a transfer sheet is decreased because of reduction of the hardness of an elastic member of a heating member due to thermal degradation, a sufficient quantity of heat is supplied to the transfer sheet. As a result, reduction of the fixing property and that of the reproducibility of a transparent color which are respectively caused by reduction of the hardness of the elastic member in the cases where the transfer sheet is normal paper and where the transfer sheet is an OHP sheet can be prevented from occurring.

A second aspect of the invention provides a fixing device comprising: first and second fixing members each of which has an elastic member and which are abutted against each other so that said elastic members contact with each other; a heater which heats at least one of said first and second fixing members; and a controller which corrects fixing conditions in accordance with an integrated energizing time that is an integrated value of an energizing time of said heater, and information of an image to be fixed, wherein a sheet carrying an image of a developer is passed between said first and second fixing members, whereby the developer is heated and fused under a pressure exerted by said first and second fixing members, to be fixed onto the sheet.

In a fixing device of a second aspect of the invention, fixing conditions are corrected in consideration of not only the integrated energizing time but also of the image information. Therefore, reduction of the fixing property in the case where the transfer sheet is normal paper, and that of the reproducibility of a transparent color in the case where the transfer sheet is an OHP sheet can be compensated more surely. Further, as a result of the correction of the fixing conditions, an excessive heat load is prevented from being applied to the elastic members to promote advancement of the thermal degradation.

Third aspect of the invention provides a fixing device comprising: first and second fixing members each of which has an elastic member and which are abutted against each other so that said elastic members contact with each other; a heater which heats at least one of said first and second fixing members; and estimating means for estimating a degree of thermal degradation of said elastic members on the basis of an integrated energizing time that is an integrated value of an energizing time of said heater, wherein a sheet carrying an image of a developer is passed between said first and second fixing members, whereby the developer is heated and fused under a pressure exerted by said first and second fixing members, to be fixed onto the sheet.

In a fixing device of a third invention, estimating means estimates the degree of thermal degradation of the elastic member on the basis of the integrated energizing time of the heater. Therefore, thermal degradation of the elastic member can be estimated by a simple configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which:

FIG. 1 is a diagram schematically showing a fixing device of a first embodiment of the invention;

FIG. 2A is a diagram showing the fixing device in an abutting state; FIG. 2B is a diagram showing the fixing device in a separating state;

FIG. 3 is a graph showing relationships between the hardness of a silicone rubber layer 26 and an integrated energizing time;

FIG. 4 is a flowchart illustrating an operation of measuring the integrated energizing time in a controller;

FIG. 5 is a flowchart illustrating a fixing operation in the first embodiment of the invention;

FIG. 6 is a flowchart showing a subroutine of step S 5 - 2 of FIG. 5 ;

FIG. 7 is a flowchart showing a subroutine of step S 5 - 3 of FIG. 5 ;

FIG. 8 is a flowchart illustrating a fixing operation in a second embodiment of the invention;

FIG. 9 is a graph showing relationships between the integrated energizing time and a fixing temperature;

FIG. 10 is a flowchart showing a subroutine of step S 8 - 2 of FIG. 8 ;

FIG. 11 is a flowchart showing a subroutine of step S 8 - 3 of FIG. 8 ;

FIG. 12 is a graph showing another example of a fixing temperature correction in the second embodiment;

FIG. 13 is a graph showing a further example of the fixing temperature correction in the second embodiment;

FIG. 14 is a flowchart showing a fixing speed correction in a normal paper mode in a third embodiment;

FIG. 15 is a flowchart showing a fixing speed correction in an OHP sheet mode in the third embodiment;

FIG. 16 is a flowchart showing a fixing temperature correction in a normal paper mode in a fourth embodiment;

FIG. 17 is a flowchart showing a fixing temperature correction in an OHP sheet mode in the fourth embodiment;

FIG. 18 is a diagram schematically showing an abutting state of a fixing device of a fifth embodiment;

FIG. 19 is a diagram schematically showing a separating state of the fixing device of the fifth embodiment;

FIG. 20 is a flowchart illustrating the operation of the fifth embodiment;

FIG. 21 is a graph illustrating correction of an integrated energizing time;

FIG. 22 is a diagram schematically showing a fixing device which is a sixth embodiment of the invention;

FIG. 23 is a graph showing relationships between a fixing temperature and glossiness;

FIG. 24 is a graph showing relationships between an integrated energizing time and transparency of an OHP sheet;

FIG. 25 is a flowchart illustrating an operation of measuring the integrated energizing time in a controller;

FIG. 26 is a flowchart illustrating the operation of the fixing device of the sixth embodiment;

FIG. 27A is a graph showing relationships between an image density and a distribution ratio;

FIG. 27B is a graph showing a factor value curve of the image density;

FIG. 27C is a graph showing relationships between an image area and the distribution ratio;

FIG. 27D is a graph showing relationships between an image area and a factor value curve;

FIG. 28 is a flowchart showing a subroutine of step S 26 - 8 of FIG. 26 ;

FIG. 29 is a flowchart showing a subroutine of step S 26 - 11 of FIG. 26 ;

FIG. 30 is a flowchart illustrating the operation of a fixing device of a seventh embodiment;

FIG. 31 is a flowchart showing a subroutine of step S 30 - 9 of FIG. 30 ;

FIG. 32 is a flowchart showing a subroutine of step S 30 - 12 of FIG. 30 ;

FIG. 33 is a diagram schematically showing a fixing device which is an eighth embodiment of the invention;

FIG. 34 is a block diagram showing the configuration of thermal degradation estimating means 231 disposed in the fixing device of the eighth embodiment;

FIG. 35 is a flowchart illustrating the operation of the thermal degradation estimating means 231 of the eighth embodiment;

FIG. 36 is a graph showing relationships between an estimated degradation time and a degradation level;

FIG. 37 is a block diagram showing thermal degradation estimating means 231 disposed in a fixing device of the ninth embodiment of the invention;

FIG. 38 is a flowchart illustrating the operation of the thermal degradation estimating means 231 of the ninth embodiment;

FIG. 39 is a flowchart showing a subroutine of step S 38 - 3 of FIG. 38 ;

FIG. 40 is a block diagram showing thermal degradation estimating means 231 disposed in a fixing device of the tenth embodiment of the invention;

FIG. 41 is a flowchart showing a subroutine of step S 38 - 3 of FIG. 38A in the tenth embodiment;

FIG. 42 is a block diagram showing thermal degradation estimating means 231 disposed in a fixing device of the eleventh embodiment of the invention;

FIG. 43 is a flowchart showing a subroutine of step S 38 - 3 of FIG. 38A in the eleventh embodiment;

FIG. 44 is a block diagram showing thermal degradation estimating means 231 disposed in a fixing device of the twelfth embodiment of the invention;

FIG. 45 is a flowchart illustrating the operation of the thermal degradation estimating means 231 of the twelfth embodiment; and

FIG. 46 is a diagram showing a conventional fixing device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

A fixing device of an embodiment of the invention shown in FIGS. 1 and 2 comprises an upper frame 12 and a lower frame 13 which are coupled to each other by an axial support pin 11 . A fixing roller 14 and a pressure roller 15 which serve as fixing members are respectively supported by the upper and lower frames 12 and 13 in a rotatable manner.

An eccentric cam 18 that is driven by a motor 17 is abutted against the lower frame 13 . By changing angles of rotation of the eccentric cam 18 , it is enabled to switch between an abutted condition in which the fixing roller 14 and pressure roller 15 are abutted against each other as shown in FIG. 2A , and a separate condition in which the fixing roller 14 and the pressure roller 15 are separated from each other as shown in FIG. 2 B. Further, by adjusting the angle of rotation of the cam 18 , a distance X between axes of the fixing roller 14 and pressure roller 15 (as shown in FIG. 2A ) can be maintained constant and thus to maintain a width of a later-mentioned nip portion 32 constant.

As shown in FIG. 1 , in addition to the fixing roller 14 , a cleaning roller 19 , an oil applying mechanism 20 , a separator 21 , and the like are disposed in the upper frame 12 . In the lower frame 13 , a temperature sensor 33 which detects the surface temperature of a silicone rubber layer 26 of the pressure roller 15 , a pre-fixing guide 22 , an oil-scraping blade 23 , a separator 24 , and the like are disposed.

In each of the fixing roller 14 and the pressure roller 15 , the silicone rubber layer 26 serving as an elastic member is disposed on the outer periphery of a metal cylinder 25 . The silicone rubber layer 26 is set to have a heat conductivity which is not smaller than 1.2×10 −3 (cal/cm·sec·° C. ) and not larger than 2.4×10 −3 (cal/cm·sec·° C.). When evaluated in accordance with JIS C 2123, the silicone rubber layer 26 has a hardness of 70 or less.

A heater 27 serving as heating means is accommodated in each of the metal cylinders 25 . A motor 28 is coupled to the pressure roller 15 . When the motor 28 is operated under the pressing state shown in FIG. 2A , the pressure roller 15 is rotated in a counterclockwise direction in the figures and the fixing roller 14 is rotated in a following manner.

A transfer sheet 30 such as normal paper or an OHP sheet in which a developer image is formed on the upper face in the figure is supplied to the nip portion 32 , and then transported to the left side in the figure by the rotation of the fixing roller 14 and the pressure roller 15 . During this process, the developer on the transfer sheet 30 is heated and fused by the heat supplied from the silicone rubber layers 26 , and then fixed onto the transfer sheet 30 by the abutting force exerted between the fixing roller 14 and the pressure roller 15 .

Referring to FIG. 1 , 35 denotes a controller constituting the estimating means in the invention. The motors 17 and 28 , the heaters 27 , and the temperature sensor 33 are connected to the controller 35 .

In the fixing device of the first embodiment, the content of a high thermal conductive filler in the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 is increased so that the heat conductivity is set to a large value or not smaller than 1.2×10 −3 (cal/cm·sec·° C.) and not larger than 2.4×10 −3 (cal/cm·sec·° C.) as described above. Therefore, as shown in FIG. 3 , the hardness of each roller is reduced as the integrated value of the energizing time (integrated energizing time) of the heaters 27 is increased. As described above, the axis-to-axis distance X between the fixing roller 14 and the pressure roller 15 is set to be constant. Thus, even when the roller hardness is reduced, the width of the nip portion 32 is constant. However, as shown in Table 1 below, the abutting force which is produced by the elasticity of the fixing roller 14 and the pressure roller 15 and acts on the nip portion 32 is decreased as a result of temporal reduction of the hardness.

[Table 1]

Relationships between the integrated energizing time and the abutting force between the fixing roller and the pressure roller

Integrated           Abutting force:
energizing time (Hr) total pressure (Kg)
0˜200                130
200˜800              95
800˜1500             85
1500˜3000            77
3000˜5000            72
5000˜                66

The decreased abutting force causes the fixing property to be reduced in the cases where the transfer sheet 30 is normal paper, and the reproducibility of a transparent color to be reduced in the case where the transfer sheet 30 is an OHP sheet. In the first embodiment, in order to eliminate an effect of such reduction of the hardness due to thermal degradation, the controller 35 implements the operations shown in the flowcharts of FIGS. 4 to 7 .

First, the controller 35 performs the measurement of the energizing time as shown in FIG. 4 .

If it is judged in step S 4 - 1 that the fixing operation is being performed, and in step S 4 - 2 that the heater 27 is being energized, then in step S 4 - 3 the temperature sensor 33 detects the temperature of the silicone rubber layer 26 of the pressure roller 15 .

If it is judged in step S 4 - 4 that the detected temperature t is not lower than a predetermined temperature t const , then in step S 4 - 5 a timer is started. The time measurement is continued until no energization is detected in step S 4 - 6 . If no energization is detected in step S 4 - 6 , then the timer is stopped in step S 4 - 7 , and the energizing time TM in this measurement is read out in step S 4 - 8 . Further, in step S 4 - 9 , the integrated value of the energizing time TM (integrated energizing time ATM) is calculated, and the integrated energizing time ATM is stored in step S 4 - 10 .

The fixing operation shown in FIG. 5 will be described. First, it is judged in step S 5 - 1 whether the copy mode is a normal paper mode or an OHP sheet mode.

If it is judged in step S 5 - 1 that the copy mode is the normal paper mode, then the control is advanced to step S 5 - 2 to perform a correction of fixing speed for the normal paper mode. In step S 5 - 2 , on the basis of Table 2 listed below, the fixing speed is corrected in accordance with the integrated energizing time ATM.

TABLE 2
Correction of fixing speed (normal paper mode)
Integrated energizing
time ATM (Hr) Correction of fixing speed
0˜200         Not corrected
200˜800       Fixing speed is decreased by 2%
800˜2000      Fixing speed is decreased by 4%
2000˜5000     Fixing speed is decreased by 6%
5000˜         Fixing speed is decreased by 8%

First, the integrated energizing time ATM is checked in step S 6 - 1 of FIG. 6 . If it is judged in step S 6 - 2 that the integrated energizing time ATM is shorter than 200 hours, then it is determined in step S 6 - 3 that the fixing speed is not corrected. If it is judged in step S 6 - 2 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 6 - 4 . If it is judged in step S 6 - 4 that the integrated energizing time ATM is shorter than 800 hours, then the fixing speed is decreased by 2% in step S 6 - 5 . If it is judged in step S 6 - 4 that the integrated energizing time ATM is not shorter than 800 hours, then the control is advanced to step S 6 - 6 . If it is judged in step S 6 - 6 that the integrated energizing time ATM is shorter than 2,000 hours, then the fixing speed is decreased by 4% in step S 6 - 7 . If it is judged in step S 6 - 6 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 6 - 8 . If it is judged in step S 6 - 8 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing speed is decreased by 6% in step S 6 - 9 . If the integrated energizing time is not shorter than 5,000 hours, then the fixing speed is decreased by 8% in step S 6 - 10 .

By contrast, if it is judged in step S 5 - 1 of FIG. 5 that the copy mode is the OHP sheet mode, then the control is advanced to step S 5 - 3 to perform a correction of fixing speed for the OHP sheet mode. In step S 5 - 3 , on the basis of Table 3 listed below, the fixing speed is corrected in accordance with the integrated energizing time ATM.

TABLE 3
Correction of fixing speed (OHP sheet mode)
Integrated energizing
time ATM (Hr) Correction of fixing speed
0˜200         Not corrected
200˜800       Fixing speed is decreased by 3%
800˜2000      Fixing speed is decreased by 6%
2000˜5000     Fixing speed is decreased by 9%
5000˜         Fixing speed is decreased by 12%

First, the integrated energizing time ATM is checked in step S 7 - 1 of FIG. 7 . If it is judged in step S 7 - 2 that the integrated energizing time ATM is shorter than 200 hours, then it is determined in step S 7 - 3 that the fixing speed is not corrected. If it is judged in step S 7 - 2 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 7 - 4 . If it is judged in step S 7 - 4 that the integrated energizing time ATM is shorter than 800 hours, then the fixing speed is decreased by 3% in step S 7 - 5 . If it is judged in step S 7 - 4 that the integrated energizing time ATM is not shorter than 800 hours, then the control is advanced to step S 7 - 6 . If it is judged in step S 7 - 6 that the integrated energizing time ATM is shorter than 2,000 hours, the fixing speed is decreased by 6% in step S 7 - 7 . If it is judged in step S 7 - 6 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 7 - 8 . If it is judged in step S 7 - 8 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing speed is decreased by 9% in step S 7 - 9 . If the integrated energizing time is not shorter than 5,000 hours, then the fixing speed is decreased by 12% in step S 7 - 10 .

As described above, the fixing speed is corrected in accordance with the integrated energizing time ATM, and the conditions of correction are changed in different manners according to the kind of the transfer sheet. Thereafter, the fixing operation is performed in step S 5 - 4 of FIG. 5 .

In the first embodiment, since the heat conductivity of each of the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 is set to a large value or not smaller than 1.2×10 −3 (cal/cm·sec·° C.) and not larger than 2.4×10 −3 (cal/cm·sec·° C.) as described above, heat is efficiently transferred from the heaters 27 to the surfaces of the silicone rubber layers 26 . Therefore, the surface temperatures of the silicone rubber layers 26 are prevented from being temporarily decreased during a period when an operation of continuously supplying sheets is started, and the fixing speed can be set to be high.

In the first embodiment, as the integrated energizing time ATM is longer, i.e., as the reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation is further advanced, the fixing speed is further decreased as described above. When the fixing speed is decreased, the time when the transfer sheet 30 stays in the nip portion 32 between the fixing roller 14 and the pressure roller 15 is prolonged, so that the quantity of heat supplied from the fixing roller 14 and the pressure roller 15 to the transfer sheet 30 is increased. Therefore, it is possible to compensate for insufficiency of the heat supplied to the transfer sheet 30 which is caused by the reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation, and which inevitably occurs in the case where the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 are set to have a high heat conductivity as described above. Consequently, the fixing device of the first embodiment can attain an excellent fixing property in the case of normal paper, and an excellent reproducibility of a transparent color in the case of an OHP sheet.

Second Embodiment

Next, a second embodiment of the invention will be described.

In the second embodiment, an effect of decrease of the quantity of heat supplied to the transfer sheet 30 which is caused by the reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation is compensated by adjusting the surface temperature of each of the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 (fixing temperature).

The mechanical structure of the fixing device of the second embodiment is substantially identical with that of the first embodiment shown in FIG. 1 , and the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 have a high heat conductivity which is not smaller than 1.2×10 −3 (cal/cm·sec·° C.) and not larger than 2.4×10 −3 (cal/cm·sec·° C.) as same as the first embodiment. The fixing temperature before correction is 160° C. in both the cases where the transfer sheet is normal paper and where the transfer sheet is an OHP sheet.

The fixing operation shown in FIG. 8 will be described. If it is judged in step S 8 - 1 that the copy mode is the normal paper mode, then the control is advanced to step S 8 - 2 to perform a correction of fixing temperature for the normal paper mode. In step S 8 - 2 , on the basis of FIG. 9 and Table 4 listed below, the fixing temperature is corrected in accordance with the integrated energizing time ATM.

TABLE 4
Correction of fixing temperature (OHP sheet mode)
Integrated energizing
time ATM (Hr) Correction of fixing temperature
0˜200         Not corrected
200˜800       Fixing temperature is increased by 1.5° C.
800˜2000      Fixing temperature is increased by 3° C.
2000˜5000     Fixing temperature is increased by 4.5° C.
5000˜         Fixing temperature is increased by 6° C.

First, the integrated energizing time ATM is checked in step S 10 - 1 of FIG. 10 . If it is judged in step S 10 - 2 that the integrated energizing time ATM is shorter than 200 hours, then it is determined in step S 10 - 3 that the fixing temperature is not corrected. If it is judged in step S 10 - 2 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 10 - 4 . If it is judged in step S 10 - 4 that the integrated energizing time ATM is shorter than 800 hours, then the fixing temperature is increased by 1.5° C. in step S 10 - 5 . If it is judged in step S 10 - 4 that the integrated energizing time ATM is not shorter than 800 hours, then the control is advanced to step S 10 - 6 . If it is judged in step S 10 - 6 that the integrated energizing time ATM is shorter than 2,000 hours, then the fixing temperature is increased by 3° C. in step S 10 - 7 . If it is judged in step S 10 - 6 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 10 - 8 . If it is judged in step S10-8 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing temperature is increased by 4.5° C. in step S 10 - 9 . If the integrated energizing time is not shorter than 5,000 hours, then the fixing temperature is increased by 6° C. in step S 10 - 10 .

By contrast, if it is judged in step S 8 - 1 of FIG. 8 that the copy mode is the OHP sheet mode, then the control is advanced to step S 8 - 3 to perform a correction of fixing temperature for the OHP sheet mode. In step S 8 - 3 , on the basis of FIG. 9 and Table 5 listed below, the fixing temperature is corrected in accordance with the integrated energizing time ATM.

TABLE 5
Correction of fixing temperature (OHP sheet mode)
Integrated energizing
time ATM (Hr) Correction of fixing temperature
0˜200         Not corrected
200˜800       Fixing temperature is increased by 2.5° C.
800˜2000      Fixing temperature is increased by 5.0° C.
2000˜5000     Fixing temperature is increased by 7.5° C.
5000˜         Fixing temperature is increased by 10° C.

First, the integrated energizing time ATM is checked in step S 11 - 1 of FIG. 11 . If it is judged in step S 11 - 2 that the integrated energizing time ATM is shorter than 200 hours, then it is determined in step S 11 - 3 that the fixing temperature is not corrected. If it is judged in step S 11 - 2 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 11 - 4 . If it is judged in step S 11 - 4 that the integrated energizing time ATM is shorter than 800 hours, then the fixing temperature is increased by 2.5° C. in step S 11 - 5 . If it is judged in step S 11 - 4 that the integrated energizing time ATM is not shorter than 800 hours, then the control is advanced to step S 11 - 6 . If it is judged in step S 11 - 6 that the integrated energizing time ATM is shorter than 2,000 hours, then the fixing temperature is increased by 5.0° C. in step S 11 - 7 . If it is judged in step S 11 - 6 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 11 - 8 . If it is judged in step S 11 - 8 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing temperature is increased by 7.5° C. in step S 11 - 9 . If the integrated energizing time ATM is not shorter than 5,000 hours, then the fixing temperature is increased by 10° C. in step S 11 - 10 .

As described above, the fixing temperature is corrected in accordance with the integrated energizing time ATM, and the conditions of the correction are changed in different manners according to the kind of the transfer sheet. Thereafter, the fixing operation is performed in step S 8 - 4 of FIG. 8 . In the second embodiment, since the heat conductivity of each of the silicone rubber layers 26 is set to a large value or not smaller than 1.2×10 −3 (cal/cm·sec·° C.) and not larger than 2.4×10 −3 (cal/cm·sec·° C.) in the same manner as the first embodiment, the efficiency of heat transfer to the surfaces of the silicone rubber layers 26 is excellent. Therefore, the surface temperatures of the silicone rubber layers 26 are prevented from being temporarily decreased during a period when an operation of continuously supplying sheets is started, and the fixing speed can be set to be high.

In the second embodiment, as the integrated energizing time ATM is longer, i.e., as the reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation is further advanced, the fixing temperature is further increased as described above. When the fixing temperature is increased, the quantity of heat supplied from the fixing roller 14 and the pressure roller 15 to the transfer sheet 30 is increased. Therefore, it is possible to compensate for insufficiency of the heat supplied to the transfer sheet 30 which is caused by the reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation, and which inevitably occurs in the case where the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 are set to have a high heat conductivity as described above. Consequently, the fixing device of the second embodiment can attain an excellent fixing property in the case of normal paper, and an excellent reproducibility of a transparent color in the case of an OHP sheet.

In the second embodiment, as shown in FIG. 9 , the fixing temperature is set to a single value in both the normal paper mode and the OHP sheet mode, and the fixing temperature is increased at constant time intervals. Alternatively, means for switching over the fixing temperature to plural set values may be disposed, and, as shown in FIG. 12 , the time interval of the operation of raising the temperature in the case of a higher set value t1 (for example, the time interval from the integrated energizing time ATM 1 to the integrated energizing time ATM 3 shown in FIG. 12 ) may be set to be shorter than that of the operation of raising the temperature in the case of a lower set value t2 (for example, the time interval from the integrated energizing time ATM 2 to the integrated energizing time ATM 4 shown in FIG. 12 ). As the fixing temperature is higher, the reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation is advanced more rapidly. Therefore, when the fixing temperature is corrected as shown in FIG. 12 , the reduction of the fixing property in normal paper and that of reproducibility of a transparent color in an OHP sheet which are caused by thermal degradation can be prevented more surely from occurring.

Alternatively, as shown in FIG. 13 , the step width “a” of the fixing temperature in one operation of correcting the fixing temperature may be set to be larger as the set value of the fixing temperature is higher. As the fixing temperature is higher, the reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation is advanced more rapidly. Therefore, when the fixing temperature is corrected as shown in FIG. 13 , insufficiency of heat supplied to the elastic member due to the thermal degradation can be compensated, so that the reduction of the fixing property in normal paper and that of reproducibility of a transparent color in an OHP sheet can be prevented more surely from occurring.

Third Embodiment

A third embodiment of the invention is configured in the same manner as the first embodiment except that the correction of the fixing speed in the normal paper mode in step S 5 - 2 of FIG. 5 and that of the fixing speed in the OHP sheet mode in step S 5 - 3 are performed differently from the first embodiment.

FIG. 14 shows the correction of fixing speed in the normal paper mode in the third embodiment. Referring to FIG. 14 , steps S 14 - 1 to S 14 - 10 are identical with steps S 6 - 1 to S 6 - 10 of the first embodiment shown in FIG. 6 . First, the correction amount of the fixing speed is determined on the basis of only the integrated energizing time ATM.

Next, in step S 14 - 11 , the controller 35 reads out the detected temperature of the temperature sensor 33 . In step S 14 - 12 , a difference Δt between the detected temperature t and a set value t SET of the fixing temperature (160° C. in this embodiment) is calculated.

Next, in steps S 14 - 13 to S 14 - 21 , the correction amount of the fixing speed is corrected in accordance with the temperature difference Δt.

If it is judged in step S 14 - 13 that the integrated energizing time ATM is shorter than 200 hours, then the fixing speed is further corrected in step S 14 - 14 in accordance with the temperature difference Δt and on the basis of Table 6 listed below.

TABLE 6
In the case where the integrated energizing time ATM is 0 to 200 hours
Temperature	0˜−2.5	−2.5˜−5	Equal to or
difference Δt(° C.)			larger than −5
Correction amount	Not	Fixing speed is	Fixing speed is
of fixing speed	corrected	decreased by 3.5%	decreased by 7%

In step S 14 - 14 , if the temperature difference Δt is within the range of 0 to −2.5° C., then the fixing speed is not corrected, and the fixing operation is performed at the fixing speed which has been corrected in steps S 14 - 1 to S 14 - 10 . If the temperature difference Δt is within the range of −2.5 to −5° C., then the fixing speed is further decreased by 3.5%, and, if the temperature difference Δt is equal to or higher than −5° C., then the fixing speed is further decreased by 7%.

If it is judged in step S 14 - 13 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 14 - 15 . If it is judged in step S 14 - 15 that the integrated energizing time ATM is shorter than 800 hours, then the fixing speed is corrected in step S 14 - 16 in accordance with the temperature difference Δt and on the basis of Table 7 listed below.

TABLE 7
In the case where the integrated energizing time ATM is 200 to 800 hours
Temperature	0˜−2.5	−2.5˜−5	Equal to or
difference Δt(° C.)			larger than −5
Correction amount	Fixing speed	Fixing speed is	Fixing speed is
of fixing speed	is decreased	decreased by	decreased by 9%
	by 2%	5.5%

In step S 14 - 16 , if the temperature difference Δt is within the range of 0 to −2.5° C., then the fixing speed is further decreased by 2%, and, if the temperature difference Δt is within the range of −2.5 to −5° C., then the fixing speed is further decreased by 5.5%, and, if the temperature difference Δt is equal to or larger than −5° C., then the fixing speed is further decreased by 9%.

If it is judged in step S 14 - 15 that the integrated energizing time ATM is not shorter than 800 hours, the control is advanced to step S 14 - 17 . If it is judged in step S 14 - 17 that the integrated energizing time ATM is shorter than 2,000 hours, the fixing speed is corrected in step S 14 - 18 in accordance with the temperature difference At and on the basis of Table 8 below.

TABLE 8
(In the case where the integrated energizing time
ATM is 800 to 2000 hours)
Temperature	0˜−2.5	−2.5˜−5	Equal to or
ditference Δt(° C.)			larger than −5
Correction amount	Fixing speed is	Fixing speed is	Fixing speed is
of fixing speed	decreased by	decreased by	decreased by
	4%	7.5%	11%

In step S 14 - 18 , if the temperature difference Δt is within the range of 0 to −2.5° C., then the fixing speed is further decreased by 4%, and, if the temperature difference At is within the range of −2.5 to −5° C., then the fixing speed is further decreased by 7.5%, and, if the temperature difference Δt is equal to or larger than −5° C., the fixing speed is further decreased by 11%.

If it is judged in step S 14 - 17 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 14 - 19 . If it is judged in step S 14 - 19 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing speed is corrected in step S 14 - 20 in accordance with the temperature difference Δt and on the basis of Table 9 listed below. By contrast, if it is judged in step S 14 - 19 that the integrated energizing time ATM is not shorter than 5,000 hours, then the fixing speed is corrected in step S 14 - 21 in accordance with the temperature difference Δt and on the basis of Table 10 listed below.

TABLE 9
In the case where the integrated energizing time
ATM is 2000 to 5000 hours
Temperature	0˜−2.5	−2.5˜−5	Equal to or
ditference Δt(° C.)			larger than −5
Correction amount	Fixing speed is	Fixing speed is	Fixing speed is
of fixing speed	decreased by	decreased by	decreased by
	6%	9.5%	13%

TABLE 10
In the case where the integrated energizing time ATM is
equal to or longer than 5,000 hours
Temperature	0˜−2.5	−2.5˜−5	Equal to or
difference Δt(° C.)			larger than −5
Correction amount	Fixing speed is	Fixing speed is	Fixing speed is
of fixing speed	decreased by	decreased by	decreased by 15%
	8%	11.5%

In step S 14 - 20 , if the temperature difference Δt is within the range of 0 to −2.5° C., then the fixing speed is further decreased by 6%, and, if the temperature difference Δt is within the range of −2.5 to −5° C., then the fixing speed is further decreased by 9.5%, and, if the temperature difference Δt is equal to or larger than −5° C., then the fixing speed is further decreased by 13%.

In step S 14 - 21 , if the temperature difference Δt is within the range of 0 to −2.5° C., then the fixing speed is further decreased by 8%, and, if the temperature difference Δt is within the range of −2.5 to −5° C., the fixing speed is further decreased by 11.5%, and, if the temperature difference Δt is equal to or larger than −5° C., the fixing speed is further decreased by 15%.

In the Tables 6 to 10 listed above, the fixing speed is decreased more largely as the temperature difference Δt is larger. As seen from comparison of Tables 6 to 10, the reduction amounts of the fixing speed respectively corresponding to the ranges of the temperature difference Δt of 0 to −2.5° C., −2.5 to −5° C., and not lower than −5° C. are larger as the integrated energizing time ATM is longer. As the temperature difference Δt between the set value of the fixing temperature and the detected temperature t of the temperature sensor 33 is larger, the actual surface temperatures of the silicone rubber layers 26 are higher and the reduction of elasticity due to the thermal degradation is further advanced. Therefore, when the fixing speed is decreased in accordance with the degree of the temperature difference Δt as described above, the reduction of the fixing property caused by the thermal degradation of the silicone rubber layers 26 can be prevented more surely from occurring. In the third embodiment, since the reduction amounts of the fixing speed respectively corresponding to the ranges of the temperature difference Δt are made different in accordance with the integrated energizing time ATM, the reduction of the fixing property can be prevented more surely from occurring.

FIG. 15 shows the correction of fixing speed in the OHP sheet mode in the third embodiment. Referring to FIG. 15 , steps S 15 - 1 to S 15 - 10 are identical with steps S 7 - 1 to S 7 - 10 of the first embodiment shown in FIG. 7 . First, the correction amount of the fixing speed is determined on the basis of only the integrated energizing time ATM.

Next, in step S 15 - 11 , the controller 35 reads out the detected temperature t of the temperature sensor 33 . In step S 15 - 12 , the temperature difference Δt is calculated.

Next, in steps S 15 - 13 to S 15 - 21 , the correction amounts of the fixing speed and the fixing temperature are corrected in accordance with the temperature difference Δt.

If it is judged in step S 15 - 13 that the integrated energizing time ATM is shorter than 200 hours, then the fixing speed is corrected in step S 15 - 14 in accordance with the temperature difference Δt and on the basis of Table 11 listed below.

TABLE 11
In the case where the integrated energizing time ATM is 0 to 200 hours
Temperature	0˜−2.5	−2.5˜−5	Equal to or
difference Δt(° C.)			larger than −5
Correction amount	Not Corrected	Fixing speed is	Fixing speed is
of fixing speed		decreased by	decreased by 10%
		5%

In step S 15 - 14 , if the temperature difference Δt is within the range of 0 to −2.5° C., then the fixing speed is not corrected, and the fixing operation is performed at the fixing speed which has been corrected in steps S 15 - 1 to S 15 - 10 . If the temperature difference Δt is within the range of −2.5 to −5° C., the fixing speed is further decreased by 5%, and, if the temperature difference Δt is equal to or larger than −5° C., then the fixing speed is further decreased by 10%.

If it is judged in step S 15 - 13 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 15 - 15 . If it is judged in step S 15 - 15 that the integrated energizing time ATM is shorter than 800 hours, then the fixing speed and the fixing temperature are corrected in step S 15 - 16 in accordance with the temperature difference Δt and on the basis of Table 12 listed below.

TABLE 12
In the case where the integrated energizing time ATM is
200 to 800 hours
Temperature	0˜−2.5	−2.5˜−5	Equal to or
difference Δt(° C.)			larger than −5
Correction amount	Fixing	Fixing	Fixing
of fixing	temperature is	temperature is	temperature is
temperature	increased by	increased by	increased by
	2.5° C.	2.5° C.	2.5° C.
Correction amount		Fixing speed is	Fixing speed is
of fixing speed		decreased by	decreased by
		5.5%	10%

In step S 15 - 16 , if the temperature difference Δt is within the range of 0 to −2.5° C., then the fixing temperature is increased by 2.5° C., and, if the temperature difference Δt is within the range of −2.5 to −5° C., then the fixing temperature is increased by 2.5° C. and the fixing speed is decreased by 5.5%, and, if the temperature difference Δt is equal to or larger than −5° C., the fixing temperature is increased by 2.5° C. and the fixing speed is decreased by 10%.

If it is judged in step S 15 - 15 that the integrated energizing time ATM is not shorter than 800 hours, then the control is advanced to step S 15 - 17 . If it is judged in step S 15 - 17 that the integrated energizing time ATM is shorter than 2,000 hours, then the fixing speed and the fixing temperature are corrected in step S 15 - 18 in accordance with the temperature difference Δt and on the basis of Table 13 listed below.

TABLE 13
In the case where the integrated energizing time ATM is
800 to 2000 hours
Temperature	0˜−2.5	−2.5˜−5	Equal to or
difference Δt(° C.)			larger than −5
Correction amount	Fixing	Fixing	Fixing
of fixing	temperature is	temperature is	temperature is
temperature	increased by	increased by	increased by
	5.0° C.	5.0° C.	5.0° C.
Correction amount		Fixing speed is	Fixing speed is
of fixing speed		decreased by	decreased by
		5%	10%

In step S 15 - 18 , if the temperature difference Δt is within the range of 0 to −2.5° C., the fixing temperature is increased by 5.0° C., and, if the temperature difference Δt is within the range of −2.5 to −5° C., then the fixing temperature is increased by 5.0° C. and the fixing speed is decreased by 5%, and, if the temperature difference Δt is equal to or larger than −5° C., then the fixing temperature is increased by 5.0° C. and the fixing speed is decreased by 10%.

If it is judged in step S 15 - 17 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 15 - 19 . If it is judged in step S 15 - 19 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing speed and the fixing temperature are corrected in step S 15 - 20 in accordance with the temperature difference Δt and on the basis of Table 14 listed below. By contrast, if it is judged in step S 15 - 19 that the integrated energizing time ATM is not shorter than 5,000 hours, then the fixing speed and the fixing temperature are corrected in step S 15 - 21 in accordance with the temperature difference Δt and on the basis of Table 15 listed below.

TABLE 14
In the case where the integrated energizing time ATM is
2000 to 5000 hours
Temperature	0˜−2.5	−2.5˜−5	Equal to or
difference Δt(° C.)			larger than −5
Correction amount	Fixing	Fixing	Fixing
of fixing	temperature is	temperature is	temperature is
temperature	increased by	increased by	increased by
	7.5° C.	7.5° C.	7.5° C.
Correction amount		Fixing speed is	Fixing speed is
of fixing speed		decreased by	decreased by
		5%	10%

TABLE 14
In the case where the integrated energizing time ATM is
2000 to 5000 hours
Temperature	0˜−2.5	−2.5˜−5	Equal to or
difference Δt(° C.)			larger than −5
Correction amount	Fixing	Fixing	Fixing
of fixing	temperature is	temperature is	temperature is
temperature	increased by	increased by	increased by
	7.5° C.	7.5° C.	7.5° C.
Correction amount		Fixing speed is	Fixing speed is
of fixing speed		decreased by	decreased by
		5%	10%

In step S 15 - 20 , if the temperature difference Δt is within the range of 0 to −2.5° C., then the fixing temperature is increased by 7.5° C., and, if the temperature difference Δt is within the range of −2.5 to −5° C., then the fixing temperature is increased by 7.5° C. and the fixing speed is decreased by 5%, and, if the temperature difference Δt is equal to or larger than −5° C., then the fixing temperature is increased by 7.5° C. and the fixing speed is decreased by 10%.

In step S 15 - 21 , if the temperature difference Δt is within the range of 0 to −2.5° C., then the fixing temperature is increased by 10.0° C., and, if the temperature difference Δt is within the range of −2.5 to −5° C., then the fixing temperature is increased by 10.0° C. and the fixing speed is decreased by 5%, and, if the temperature difference Δt is equal to or larger than −5° C., then the fixing temperature is increased by 10.0° C. and the fixing speed is decreased by 10%.

In the Tables 11 to 15 above, the fixing speed is decreased more largely as the temperature difference Δt is larger. As seen from comparison of Tables 11 to 15, the reduction amounts of the fixing speed respectively corresponding to the ranges of the temperature difference Δt of 0 to −2.5° C., −2.5 to −5° C., and not lower than −5° C. are larger and the increased amounts of the fixing temperature are larger as the integrated energizing time ATM is longer. As the temperature difference Δt between the set value of the fixing temperature and the detected fixing temperature is larger, the temperatures of the silicone rubber layers 26 are higher and the reduction of elasticity due to the thermal degradation is further advanced. Therefore, when the fixing speed is decreased in accordance with the degree of the temperature difference Δt as described above, the reduction of the fixing property and that of the reproducibility of a transparent color which are caused by the thermal degradation of the silicone rubber layers 26 can be prevented more surely from occurring. Since the reduction amounts of the fixing speed and the increased amounts of the fixing temperature respectively corresponding to the ranges of the temperature difference Δt are made different in accordance with the integrated energizing time ATM, the reduction of the fixing property and that of the reproducibility of a transparent color can be prevented more surely from occurring.

Fourth Embodiment

A fourth embodiment of the invention is configured in the same manner as the second embodiment except that the correction of the fixing temperature in the normal paper mode in step S 8 - 2 of FIG. 8 and that of the fixing temperature in the OHP sheet mode in step S 8 - 3 are performed differently from the second embodiment.

FIG. 16 shows the fixing temperature correction in the normal paper mode in the fourth embodiment. Referring to FIG. 16 , steps S 16 - 1 to S 16 - 10 are identical with steps S 10 - 1 to S 10 - 10 of the second embodiment shown in FIG. 10 . First, the correction amount of the fixing temperature is determined on the basis of only the integrated energizing time ATM.

Next, in step S 16 - 11 , the controller 35 reads out the detected temperature t of the temperature sensor 33 . In step S 16 - 12 , the difference Δt between the detected temperature t and a set value t SET of the fixing temperature is calculated.

Next, in steps S 16 - 13 to S 16 - 21 , the correction amount of the fixing speed is corrected in accordance with the temperature difference Δt.

If it is judged in step S 16 - 13 that the integrated energizing time ATM is shorter than 200 hours, then the fixing speed is corrected in step S 16 - 14 in accordance with the temperature difference Δt and on the basis of Table 6 listed above.

If it is judged in step S 16 - 13 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 16 - 15 . If it is judged in step S 16 - 15 that the integrated energizing time ATM is shorter than 800 hours, then the fixing speed is corrected in step S 16 - 16 in accordance with the temperature difference Δt and on the basis of Table 7 listed above.

If it is judged in step S 16 - 15 that the integrated energizing time ATM is not shorter than 800 hours, then the control is advanced to step S 16 - 17 . If it is judged in step S 16 - 17 that the integrated energizing time ATM is shorter than 2,000 hours, then the fixing speed is corrected in step S 16 - 18 in accordance with the temperature difference Δt and on the basis of Table 8 listed above.

If it is judged in step S 16 - 17 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 16 - 19 . If it is judged in step S 16 - 19 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing speed is corrected in step S 16 - 20 in accordance with the temperature difference Δt and on the basis of Table 9 listed above. If it is judged in step S 16 - 19 that the integrated energizing time ATM is not shorter than 5,000 hours, then the fixing speed is corrected in step S 16 - 21 in accordance with the temperature difference Δt and on the basis of Table 10 listed above.

When the fixing temperature is corrected in accordance with the integrated energizing time ATM and the fixing speed is corrected in accordance with the temperature difference Δt as described above, the effect of reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation can be reduced more surely, and hence it is possible to obtain a more excellent fixing property.

FIG. 17 shows the correction of fixing temperature in the OHP sheet mode in the fourth embodiment. Referring to FIG. 17 , steps S 17 - 1 to S 17 - 10 are identical with steps S 11 - 1 to S 11 - 10 of the second embodiment shown in FIG. 11 . First, the correction amount of the fixing temperature is determined on the basis of only the integrated energizing time ATM.

Next, in step S 17 - 11 , the controller 35 reads out the detected temperature t of the temperature sensor 33 . In step S 17 - 12 , the temperature difference Δt is calculated.

Next, in steps S 17 - 13 to S 17 - 21 , the fixing speed and the fixing temperature are corrected in accordance with the temperature difference Δt.

If it is judged in step S 17 - 13 that the integrated energizing time ATM is shorter than 200 hours, then the fixing speed is corrected in step S 17 - 14 in accordance with the temperature difference Δt and on the basis of Table 11 listed above.

If it is judged in step S 17 - 13 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 17 - 15 . If it is judged in step S 17 - 15 that the integrated energizing time ATM is shorter than 800 hours, then the fixing speed and the fixing temperature are corrected in step S 17 - 16 in accordance with the temperature difference Δt and on the basis of Table 12 listed above.

If it is judged in step S 17 - 15 that the integrated energizing time ATM is not shorter than 800 hours, then the control is advanced to step S 17 - 17 . If it is judged in step S 17 - 17 that the integrated energizing time ATM is shorter than 2,000 hours, then the fixing speed and the fixing temperature are corrected in step S 17 - 18 in accordance with the temperature difference Δt and on the basis of Table 13 listed above.

If it is judged in step S 17 - 17 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 17 - 19 . If it is judged in step S 17 - 19 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing speed and the fixing temperature are corrected in step S 17 - 20 in accordance with the temperature difference Δt and on the basis of Table 14 listed above. If it is judged in step S 17 - 19 that the integrated energizing time ATM is not shorter than 5,000 hours, then the fixing speed and the fixing temperature are corrected in accordance with the temperature difference Δt and on the basis of Table 15 listed above.

When the fixing temperature is corrected in accordance with the integrated energizing time ATM, and the fixing speed and the fixing temperature are corrected in accordance with the temperature difference Δt as described above, the effect of reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation can be reduced more surely, and hence it is possible to obtain a more excellent reproducibility of a transparent color.

Fifth Embodiment FIGS. 18 and 19 show a fifth embodiment of the invention.

A spring 40 is coupled at the upper end in the figures to a lower portion of the lower frame 13 . A cam plate 41 is fixed to the lower end of the spring 40 in the figures. The cam plate 41 abuts against an eccentric cam 43 which is rotated by a motor 42 . An upward projection 13 a is disposed on an upper end portion of the lower frame 13 .

In FIGS. 18 and 19 , the components disposed in the upper frame 12 , such as the cleaning roller 19 and the oil applying mechanism 20 , and those disposed in the lower frame 13 , such as the pre-fixing guide 22 , the oil scraping blade 23 , and the separator 24 are not shown.

The spring 40 urges the lower frame 13 against the upper frame 12 . In the pressing state shown in FIG. 18 , the projection 13 a abuts a lower portion of the upper frame 12 , whereby the axis-to-axis distance X is maintained constant. As a result, also the width of the nip portion 32 is maintained constant.

In the case where, in the pressing state shown in FIG. 18 , plural overlapping transfer sheets 30 are supplied to the nip portion 32 , or the case where a foreign substance is supplied together with the transfer sheet 30 to the nip portion 32 , the force which acts on the nip portion 32 so as to separate the fixing roller 14 and the pressure roller 15 from each other becomes greater than the abutting force exerted by the spring 40 . Then, the lower frame 13 is rotated in a counterclockwise direction in the figures, so that the fixing roller 14 and the pressure roller 15 are separated from each other.

When the rotation angle of the eccentric cam 43 is finely adjusted in the pressing state shown in FIG. 18 , it is possible to control the force which is exerted by the spring 40 to press the lower frame 13 against the upper frame 12 so that the fixing roller 14 and the pressure roller 15 are abutted with each other. For example, when the eccentric cam 43 in the rotation angle position shown in FIG. 18 is rotated in the direction of the arrow X 1 , the force exerted by the spring 40 to abut the fixing roller 14 and the pressure roller 15 against each other is increased. When the eccentric cam 43 is rotated in the direction of the arrow X 2 , the force exerted by the spring 40 to abut the fixing roller 14 and the pressure roller 15 against each other is decreased.

On the other hand, when the rotation angle position of the eccentric cam 43 is set to the position shown in FIG. 19 , a separating state in which the fixing roller 14 and the pressure roller 15 are separated from each other is attained.

Next, the operation of the fifth embodiment will be described. In the same manner as the first to fourth embodiments, the controller 35 implements the operations shown in the flowchart of FIG. 4 to calculate the integrated energizing time ATM.

In accordance with the integrated energizing time ATM, the controller 35 adjusts the force of urging the lower frame 13 against the upper frame 12 . First, the controller 35 checks the integrated energizing time ATM in step S 20 - 1 of FIG. 20 .

In step S 20 - 2 , the correction amount of the pressing force is determined in accordance with the integrated energizing time ATM. As the integrated energizing time ATM is longer, the reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation is further advanced. When the hardness of the silicone rubber layer 26 is reduced, the strength of the silicone rubber layers 26 against a stress produced in the nip portion 32 between the fixing roller 14 and the pressure roller 15 is decreased.

In the case where the pressing force of the spring 40 is greater as compared with the stress of the nip portion 32 , when plural overlapping transfer sheets 30 are supplied to the nip portion 32 or a foreign substance is supplied together with the transfer sheet 30 to the nip portion 32 , an excessive load is applied to the fixing roller 14 and the pressure roller 15 and between the upper frame 12 and the lower frame 13 , thereby producing a fear that these components are broken. By contrast, in the case where the pressing force of the spring 40 is smaller as compared with the stress of the nip portion 32 , there arises a fear that a failure occurs in the operation of supplying the transfer sheet 30 .

In order to prevent the pressing force from being made greater as compared with the stress of the nip portion 32 which is reduced by the hardness reduction due to the thermal degradation, a correction amount is determined in step S 20 - 2 so that the pressing force of the spring 40 is further reduced as the integrated energizing time ATM is longer as shown in Table 16 listed below. The correction amount is set so that the pressing force is not excessively smaller with respect to the stress of the nip portion 32 .

TABLE 16
Correction of fixing pressure
Integrated energizing
time ATM (Hr) Correction of fixing pressure
0˜200         Not corrected
200˜800       Fixing pressure is decreased by 6%
800˜2000      Fixing pressure is decreased by 12%
2000˜5000     Fixing pressure is decreased by 18%
5000˜         Fixing pressure is decreased by 24%

In step S 20 - 3 , the controller 35 drives the motor 42 to adjust the rotation angle position of the eccentric cam 43 so that the pressing force of the spring 40 is reduced by the above correction amount. Thereafter, the fixing operation is performed in step S 20 - 4 .

In the fixing device of the fifth embodiment, as described above, the pressing force exerted by the spring 40 is reduced in accordance with the reduction of the hardness of the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 which is due to thermal degradation. Therefore, even when the hardness of the silicone rubber layers 26 is reduced as a result of thermal degradation, the balance between the stress of the nip portion 32 between the fixing roller 14 and the pressure roller 15 , and the pressing force of the spring 40 is maintained. Consequently, in the case where a foreign substance or the like is supplied, the fixing roller 14 and the pressure roller 15 are surely separated from each other, thereby preventing the fixing roller 14 and the pressure roller 15 from being broken. Furthermore, a failure in the sheet supplying operation due to an insufficient pressing force of the spring 40 is prevented from occurring.

In the fixing device of the fifth embodiment, in the same manner as the first to fourth embodiments, the fixing temperature and the fixing speed may be corrected in accordance with the integrated energizing time ATM and the temperature difference Δt.

The first to fifth embodiments may be configured in the following manner. As shown in FIG. 21 , when the detected value t of the fixing temperature is equal to the set temperature, the time measured by the timer (see steps S 4 - 1 to S 4 - 10 of FIG. 4 ) is integrated as it is as an energizing time TM1. When the detected value t of the fixing temperature is higher than the set temperature by 10° C. or more, a value which is obtained by multiplying the time measured by the timer with a factor that is larger than 1 is set as an energizing time TM1′. When the detected value t of the fixing temperature is lower than the set temperature, a value which is obtained by multiplying the time measured by the timer with a predetermined factor of 0 to less than 1 is set as an energizing time TM1″.

As the actual fixing temperature is higher, the reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation is further advanced. Therefore, when the time measured by the timer is corrected in accordance with the detected temperature t as described above, it is possible to estimate more accurately the reduction of the hardness of the silicone rubber layers 26 due to thermal degradation.

Sixth Embodiment

FIG. 22 shows a fixing device of a sixth embodiment of the invention. The fixing device of the sixth embodiment is identical with the first embodiment except a controller 135 . Hence like components are denoted by the same reference numerals, and the description is omitted.

The motors 117 and 28 , the heater 27 , and the temperature sensor 33 are connected to the controller 135 of the fixing device. A main controller 136 , an operation portion 137 , an image reading portion 138 , and an image forming portion 139 of the main unit of a copying machine are connected to the controller 135 . The copy mode which will be described later can be manually set through the operation portion 137 . Image information of an original which is read by the image reading portion 138 is sent to the image forming portion 139 via the main controller 136 .

In the image forming portion 139 , an electrostatic latent image is formed on an image bearing body (not shown) on the basis of the image information. The electrostatic latent image is developed by a developer, and then transferred onto the transfer sheet 30 . Thereafter, the transfer sheet 30 onto which the developer image has been transferred is supplied to the fixing device.

In the same manner as the first embodiment, the content of a high thermal conductive filler in the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 is increased so that the heat conductivity is set to a large value or not smaller than 1.2×10 −3 (cal/cm·sec·° C.) and not larger than 2.4×10 −3 (cal/cm·sec·° C.) as described above. As shown in FIG. 3 . therefore, the hardness of each roller is reduced as the integrated value of the energizing time (integrated energizing time) of the heaters 27 is increased. In accordance with this reduction, as shown in Table 1 listed above, the abutting force acting on the nip portion 32 is decreased.

The reduced abutting force causes reduction of the fixing property in the case where the transfer sheet 30 is normal paper, and that of the reproducibility of a transparent color in the case where the transfer sheet 30 is an OHP sheet. The reduction of the fixing property and that of the reproducibility of a transparent color are caused by a phenomenon in which heat supplied from the fixing roller 14 and the pressure roller 15 to the transfer sheet 30 becomes insufficient because of reduction of the hardness of the silicone rubber layers 26 due to thermal degradation. Therefore, when the quantity of heat supplied from the fixing roller 14 and the pressure roller 15 to the transfer sheet 30 is increased in accordance with the increase of the integrated energizing time, the fixing property and the reproducibility of a transparent color can be prevented from being reduced.

In the case of normal paper, as shown in FIG. 23 , the glossiness of an image on the transfer sheet 30 is increased when the surface temperature (fixing temperature) of each of the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 is increased. As the area of a high-density area is larger, a change of the glossiness due to the raising of the fixing temperature is larger.

By contrast, in the case of an OHP sheet, as shown in FIG. 24 , the reproducibility of a transparent color is more reduced as the integrated energizing time is longer. In FIG. 24 , the transparency rank of the ordinate shows that, as its value is larger, the reproducibility of a transparent color is higher. Namely, “1” indicates that the whole portion is obscured, “2” indicates that a highlight portion (a portion of a low image density) is obscured, “3” indicates that a highlight portion is slightly obscured, “4” indicates that transparency is attained, and “5” indicates that clear transparency is attained. As the area of a highlight portion of a half tone is larger, an effect to reduction of the reproducibility of a transparent color due to a prolonged integrated energizing time is larger.

When the quantity of supplied heat is simply increased in accordance with an increase of the integrated energizing time, an excessive heat load is applied to the fixing roller 14 and the pressure roller 15 .

Thus, in the sixth embodiment, the controller 135 compensates an effect of reduction of the hardness of the silicone rubber layers 26 due to the thermal degradation, in consideration of the image density and the image area. The controller 135 performs the measurement of the energizing time shown in FIG. 25 .

If it is judged in step S 25 - 1 that the fixing operation is being performed, and in step S 25 - 2 that the heater 27 is being energized, then the temperature sensor 33 detects in step S 25 - 3 the temperature of the silicone rubber layer 26 of the pressure roller 15 .

If it is judged in step S 25 - 4 that the detected temperature t is not lower than the predetermined temperature t const , then the timer is started in step S 25 - 5 . The time measurement is continued until no energization is detected in step S 25 - 6 . If no energization is detected in step S 25 - 6 , then the timer is stopped in step S 25 - 7 , and the energizing time TM in this measurement is read out in step S 25 - 8 . In step S 25 - 9 , the integrated value of the energizing time TM (integrated energizing time ATM) is calculated, and the calculated integrated energizing time ATM is stored in step S 25 - 10 .

The fixing operation shown in FIG. 26 will be described. First, in step S 26 - 1 , from the main controller 136 , the controller 135 reads out information (transfer sheet information) indicative of a selected one of the normal paper mode in which the transfer sheet 30 is normal paper, the OHP sheet mode in which the transfer sheet 30 is an OHP sheet, and a mixed paper mode in which normal paper and an OHP sheet are alternately used for the same developer.

In step S 26 - 2 , information of an image (image information) which is sent from the image reading portion 138 to the image forming portion 139 via the main controller 136 is read into the controller 135 . The image information contains the image density of each pixel. In the sixth embodiment, the pixel number is set to be 600 dpi×600 dpi=360,000 dots/inch 2 , and the image density of each pixel has 256 levels.

In step S 26 - 3 , an image information data value “Y” is calculated on the basis of the image information which has been read in step S 26 - 2 .

For example, as shown in FIG. 27A , a distribution ratio “x” for each of the image density value “a” (a=1 to 256) is calculated. Next, a factor value “α” corresponding to each of the image density value “a” is obtained from the graph of FIG. 27 B.

A distribution ratio “y” of an image area value “b” for each of the image density value “a” is calculated. For example, FIG. 27C shows the distribution ratio “y” of the image area value “b” in the case where the image density value “a” of FIG. 27A is 130. Furthermore, a factor value “β” corresponding to the distribution ratio “y” of each of the image area values “b” is obtained from the graph of FIG. 27 D. For each of the image area values “b”, the distribution ratio “y” is multiplied with the factor value “β”, and a value “δ” is calculated by integrating the result of the multiplication with respect to the image area value “b”.

The image information data value “Y” is obtained by integrating the result of a multiplication of the distribution ratio “x”, the factor value “α”, and the value “δ” with respect to the image density value “a”. The image information data value “Y” is indicated by the following equation (1).

Y=Σxαδ=x 1 α 1 δ 1 +x 2 α 2 δ 2 . . . +x 255 α 255 δ 255 +x 256 α 256 δ 256   (1)

In the equation (1), each of the suffixes of 1 to 256 attached to the distribution ratio “x”, the factor value “α”, and the value “δ” indicates the image density value “a”.

As described above, the image information data value “Y” which is calculated from the image density and the image area indicates the property of the image. When the image information data value “Y” is a relatively small value of about 2,000 or less, it indicates that the image consists of characters, a graph, or the like. When the image information data value “Y” is an intermediate value of about 2,001 to 14,000, it indicates that the image is a half-tone image having a possibility that there is a highlight portion. When the image information data value “Y” is a relatively large value of about 14,001 or more, it indicates that the image is an image having a high-density portion of a large area.

After the image information data value “Y” is calculated in step S 26 - 3 , the integrated energizing time ATM is read into the controller 135 in step S 26 - 4 .

If it is judged in step S 26 - 5 that the current mode is not the mixed paper mode, then the control is advanced to step S 26 - 7 . In step S 26 - 7 , it is judged whether the recording sheet is normal paper or an OHP sheet. If the recording sheet is normal paper, then the control is advanced to step S 26 - 8 .

In step S 26 - 8 , the fixing conditions for the normal paper mode are corrected in accordance with the integrated energizing time ATM and the image information data value “Y”. In the sixth embodiment, the fixing temperature before correction has an initial value of 160° C., and the fixing speed before correction has an initial value of 150 mm/sec. The nip width is 6.5 mm.

If it is judged in step S 28 - 1 of FIG. 28 that the integrated energizing time ATM is shorter than 200 hours, then it is determined in step S 28 - 2 that the fixing conditions are not corrected. If it is judged in step S 28 - 1 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 28 - 3 . If it is judged in step S 28 - 3 that the integrated energizing time ATM is shorter than 800 hours, then the fixing conditions are corrected in step S 28 - 4 on the basis of Table 17.

TABLE 17
Image information data value “Y” and correction of fixing conditions
(In the case where the integrated energizing time ATM is
not shorter than 200 hours and is shorter than 800 hours)
                                 Correction of fixing
Image information data value Y   conditions
˜2000                            Fixing temperature is
(image information data value Y is small: increased by 1.5
character image, graph, or the like)
2001˜14000                       Fixing speed is
(image information data value Y is decreased by 2%
intermediate:
image which may have a highlight portion)
14001˜                           Fixing speed is
(image information data value Y is large: decreased by 2%
image having a large high-density portion)

In step S 28 - 4 , if the image information data value “Y” is not larger than 2,000, the fixing temperature is increased by 1.5° C. If the image information data value “Y” is not smaller than 2,001 and not larger than 14,000, the fixing speed is decreased by 2%. Also in the case where the image information data value “Y” is not smaller than 14,001, the fixing speed is decreased by 2%.

In both the cases where the fixing temperature is increased, and where the fixing speed is decreased, the quantity of heat supplied from the nip portion 32 to the transfer sheet 30 is increased. However, with respect to the effect on the glossiness of an image, the increase of the fixing temperature exerts a larger effect than that exerted by the decrease of the fixing speed. For example, when the case where the fixing temperature is increased by 1.5° C. is compared with that where the fixing speed is decreased by 2%, the increase of the glossiness of an image in the former case is very larger than that in the latter case.

If it is judged in step S 28 - 3 that the integrated energizing time ATM is not shorter than 800 hours, then the control is advanced to step S 28 - 5 . If it is judged in step S 28 - 5 that the integrated energizing time ATM is shorter than 2,000 hours, then the fixing conditions are corrected in step S 28 - 6 on the basis of Table 18 listed below.

TABLE 18
Image information data value “Y” and correction of fixing conditions
(In the case where the integrated energizing time ATM is
not shorter than 800 hours and is shorter than 2000 hours)
                                 Correction of fixing
Image information data value Y   conditions
˜2000                            Fixing temperature is
(image information data value Y is small: increased by 3° C.
character image, graph, or the like)
2001˜14000                       Fixing temperature is
(image information data value Y is increased by 1.5° C.
intermediate:                    Fixing speed is
image which may have a highlight portion) decreased by 2%.
14001˜                           Fixing speed is
(image information data value Y is large: decreased by 4%.
image having a large high-density portion)

In step S 28 - 6 , if the image information data value “Y” is not larger than 2,000, then the fixing temperature is increased by 3° C. If the image information data value “Y” is not smaller than 2,001 and not larger than 14,000, then the fixing temperature is increased by 1.5° C. and the fixing speed is decreased by 2%. If the image information data value “Y” is not smaller than 14,001, then the fixing speed is decreased by 4%.

If it is judged in step S 28 - 5 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 28 - 7 . If it is judged in step S 28 - 7 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing conditions are corrected in step S 28 - 8 on the basis of Table 19 listed below.

TABLE 19
Image information data value “Y” and correction of fixing conditions
(In the case where the integrated energizing time ATM is
not shorter than 2000 hours and is shorter than 5000 hours)
                                 Correction of fixing
Image information data value Y   conditions
˜2000                            Fixing temperature is
(image information data value Y is small: increased by 4.5° C.
character image, graph, or the like)
2001˜14000                       Fixing temperature is
(image information data value Y is increased by 1.5° C.
intermediate:                    Fixing speed is
image which may have a highlight portion) decreased by 4%.
14001˜                           Fixing speed is
(image information data value Y is large: decreased by 6%.
image having a large high-density portion)

In step S 28 - 8 , if the image information data value “Y” is not larger than 2,000, then the fixing temperature is increased by 4.5° C. If the image information data value “Y” is not smaller than 2,001 and not larger than 14,000, then the fixing temperature is increased by 1.5° C. and the fixing speed is decreased by 4%. If the image information data value Y is not smaller than 14,001, then the fixing speed is decreased by 6%.

If it is judged in step S 28 - 7 that the integrated energizing time ATM is not shorter than 5,000 hours, then the fixing conditions are corrected in step S 28 - 9 on the basis of Table 20 listed below.

TABLE 20
Image information data value “Y” and correction of fixing conditions
(In the case where the integrated energizing time ATM is
equal to or longer than 5,000 hours)
                                 Correction of fixing
Image information data value Y   conditions
˜2000                            Fixing temperature is
(image information data value Y is small: increased by 6.0° C.
character image, graph, or the like)
2001˜14000                       Fixing temperature is
(image information data value Y is increased by 3° C.
intermediate:                    Fixing speed is
image which may have a highlight portion) decreased by 4%.
14001˜                           Fixing speed is
(image information data value Y is large: decreased by 8%.
image having a large high-density portion)

In step S 28 - 9 , if the image information data value “Y” is not larger than 2,000, the fixing temperature is increased by 6.0° C. If the image information data value “Y” is not smaller than 2,001 and not larger than 14,000, then the fixing temperature is increased by 3° C. and the fixing speed is decreased by 4%. If the image information data value “Y” is not smaller than 14,001, then the fixing speed is decreased by 8%.

After the fixing conditions are corrected, the fixing operation is performed in step S 26 - 9 of FIG. 26 . If the end of the copying operation is detected in step S 26 - 10 , then the fixing operation is ended.

As described above, in the normal paper mode, at least one of the increase of the fixing temperature and the decrease of the fixing speed is performed in accordance with the increase of the integrated energizing time ATM. When the fixing temperature is increased or the fixing speed is decreased, the quantity of heat supplied from the nip portion 32 to the transfer sheet 30 is increased, and hence the reduction of the fixing property which is caused by the reduction of the efficiency of the heat supply due to the thermal degradation of the silicone rubber layers 26 can be prevented from occurring.

As shown in Tables 17 to 20, in the case where the integrated energizing time ATM is in the same range, the increased amount of the fixing temperature and the decreased amount of the fixing speed are set to be larger as the image information data value “Y” is larger. Therefore, for an image such as a character having a small image information data value “Y”, the fixing speed is not decreased, and a high productivity is obtained. For an image which has a high-density portion and hence a large image data value “Y”, a change of glossiness due to the correction of the fixing conditions can be suppressed.

Since the fixing conditions are corrected in consideration of not only the integrated energizing time ATM but also of the image information as described above, an excessive heat load is prevented from being applied to the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 , whereby advancement of thermal degradation can be suppressed.

If it is judged in step S 26 - 7 that the recording sheet is an OHP sheet, then the fixing conditions for the OHP sheet mode are corrected in step S 26 - 11 on the basis of the integrated energizing time ATM and the image information data value “Y”.

If it is judged in step S 29 - 1 of FIG. 29 that the integrated energizing time ATM is shorter than 200 hours, then it is determined in step S 29 - 2 that the fixing conditions are not corrected. If it is judged in step S 29 - 1 that the integrated energizing time ATM is not shorter than 200 hours, then the control is advanced to step S 29 - 3 . If it is judged in step S 29 - 3 that the integrated energizing time ATM is shorter than 800 hours, then the fixing conditions are corrected on the basis of Table 21 listed below.

TABLE 21
Image information data value “Y” and correction of fixing conditions
(In the case where the integrated energizing time ATM is
not shorter than 200 hours and is shorter than 800 hours)
                                 Correction of fixing
Image information data value Y   conditions
˜2000                            Not corrected
(image information data value Y is small:
character image, graph, or the like)
2001˜14000                       Fixing temperature is
(image information data value Y is increased by 2.5° C.
intermediate:
image which may have a highlight portion)
14001˜                           Not corrected
(image information data value Y is large:
image having a large high-density portion)

In step S 29 - 4 , if the image information data value “Y” is not larger than 2,000, then the fixing conditions are not corrected. If the image information data value “Y” is not smaller than 2,001 and not larger than 14,000, then the fixing temperature is increased by 2.5° C. If the image information data value “Y” is not smaller than 14,001, the fixing conditions are not corrected.

If it is judged in step S 29 - 3 that the integrated energizing time ATM is not shorter than 800 hours, then the control is advanced to step S 29 - 5 . If it is judged in step S 29 - 5 that the integrated energizing time ATM is shorter than 2,000 hours, then the fixing conditions are corrected on the basis of Table 22 listed below.

TABLE 22
Image information data value “Y” and correction of fixing conditions
(In the case where the integrated energizing time ATM is
not shorter than 800 hours and is shorter than 2000 hours)
                                 Correction of fixing
Image information data value Y   conditions
˜2000                            Not corrected
(image information data value Y is small:
character image, graph, or the like)
2001˜14000                       Fixing temperature is
(image information data value Y is increased by 5.0° C.
intermediate:
image which may have a highlight portion)
14001˜                           Fixing temperature is
(image information data value Y is large: increased by 2.5° C.
image having a large high-density portion)

In step S 29 - 6 , if the image information data value “Y” is not larger than 2,000, the fixing conditions are not corrected. If the image information data value “Y” is not smaller than 2,001 and not larger than 14,000, then the fixing temperature is increased by 5.0° C. If the image information data value “Y” is not smaller than 14,001, then the fixing temperature is increased by 2.5° C.

If it is judged in step S 29 - 5 that the integrated energizing time ATM is not shorter than 2,000 hours, then the control is advanced to step S 29 - 7 . If it is judged in step S 29 - 7 that the integrated energizing time ATM is shorter than 5,000 hours, then the fixing conditions are corrected in step S 29 - 8 on the basis of Table 23 listed below.

TABLE 23
Image information data value “Y” and correction of fixing conditions
(In the case where the integrated energizing time ATM is
not shorter than 2000 hours and is shorter than 5000 hours)
                                 Correction of fixing
Image information data value Y   conditions
˜2000                            Fixing temperature is
(image information data value Y is small: increased by 2.5° C.
character image, graph, or the like)
2001˜14000                       Fixing temperature is
(image information data value Y is increased by 7.5° C.
intermediate:
image which may have a highlight portion)
14001˜                           Fixing temperature is
(image information data value Y is large: increased by 5.0° C.
image having a large high-density portion)

In step S 29 - 8 , if the image information data value “Y” is not larger than 2,000, then the fixing temperature is increased by 2.5° C. If the image information data value “Y” is not smaller than 2,001 and not larger than 14,000, then the fixing temperature is increased by 7.5° C. If the image information data value “Y” is not smaller than 14,001, then the fixing temperature is increased by 5.0° C.

If it is judged in step S 29 - 7 that the integrated energizing time ATM is not shorter than 5,000 hours, then the fixing conditions are corrected in step S 29 - 9 on the basis of Table 24 listed below.

TABLE 24
Image information data value “Y” and correction of fixing conditions
(In the case where the integrated energizing time ATM is
equal to or longer than 5,000 hours)
                                 Correction of fixing
Image information data value Y   conditions
˜2000                            Fixing temperature is
(image information data value Y is small: increased by 5.0° C.
character image, graph, or the like)
2001˜14000                       Fixing temperature is
(image information data value Y is increased by 10° C.
intermediate:
image which may have a highlight portion)
14001˜                           Fixing temperature is
(image information data value Y is large: increased by 7.5° C.
image having a large high-density portion)

In step S 29-9 , if the image information data value “Y” is not larger than 2,000, then the fixing temperature is increased by 5.0° C. If the image information data value “Y” is not smaller than 2,001 and not larger than 14,000, then the fixing temperature is increased by 10° C. If the image information data value “Y” is not smaller than 14,001, then the fixing temperature is increased by 7.5° C.

After the fixing conditions are corrected, the fixing operation is performed in step S 26 - 12 of FIG. 26 . If the end of the copying operation is detected in step S 26 - 13 , then the fixing operation is ended.

As described above, in the OHP sheet mode, the increase of the fixing temperature is performed in accordance with the increase of the integrated energizing time ATM, thereby increasing the amount of heat supplied to the transfer sheet 30 . Therefore, the reduction of the reproducibility of a transparent color which is caused by the reduction of the efficiency of the heat supply due to the thermal degradation of the silicone rubber layers 26 can be prevented from occurring. As a result, the fixing speed can be set to be higher, so that a high productivity is obtained.

As shown in Tables 21 to 24, in the case where the integrated energizing time ATM is in the same range, the increased amount of the fixing temperature is set to be larger in the sequence of the cases where the image information data value “Y” is intermediate (not smaller than 2,001 and not larger than 14,000), where the image information data value “Y” is large (not smaller than 14,001), and where the image information data value “Y” is small (not larger than 2,000). As described above, the reduction of the reproducibility of a transparent color which is caused by the reduction of the efficiency of the heat supply due to the thermal degradation of the silicone rubber layers 26 is most conspicuous in the case of an image having a possibility that there is a highlight portion. Therefore, when the fixing conditions are corrected as described above with respect to the image information data value “Y”, the reduction of the reproducibility of a transparent color can be prevented more surely from occurring.

When the fixing conditions are corrected as described above with respect to the image information data value “Y”, an excessive heat load is prevented from being applied to the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 , whereby advancement of thermal degradation can be suppressed.

If it is judged in step S 26 - 5 of FIG. 26 that the current mode is the mixed paper mode, then the correction conditions are selected in step 26 - 6 . In step 26 - 6 , in the cases where the image information data value “Y” is small (not larger than 2,000) and where the image information data value “Y” is large (not smaller than 14,001), the correction conditions for the normal paper mode are selected, and the control is advanced to step S 26 - 8 to perform correction of the fixing conditions under the same conditions as those in the normal paper mode shown in FIG. 28 . By contrast, in step 26 - 6 , in the case where the image information data value “Y” is intermediate (not smaller than 2,001 and not larger than 14,000), the correction conditions for the OHP sheet mode are selected, and the control is advanced to step S 26 - 11 to perform correction of the fixing conditions under the same conditions as those in the OHP sheet mode shown in FIG. 29 .

As described above, in the case where the image information data value “Y” is intermediate, the thermal degradation of the silicone rubber layers 26 exerts the greatest effect on the reproducibility of a transparent color in the OHP sheet mode, and, in the case where the image information data value “Y” is large, the thermal degradation of the silicone rubber layers 26 exerts the greatest effect on the change of glossiness in the normal paper mode. Therefor, in the mixed paper mode as described above, the fixing conditions are corrected while, in the case where the image information data value “Y” is intermediate, emphasis is placed on prevention of reduction of the reproducibility of a transparent color, and, in the case where the image information data value Y is large, emphasis is placed on prevention of reduction of the fixing property, whereby both the reproducibility of a transparent color and the fixing property are improved.

In the case where the image information data value “Y” is small, the change of the heat supplying efficiency due to the thermal degradation of the silicone rubber layers 26 does not exert a particularly large effect on one of the fixing property in the normal paper mode, and the reproducibility of a transparent color in the OHP sheet mode. Therefore, in the case where the image information data value “Y” is small, the fixing conditions may be corrected under the same conditions as those in the OHP sheet mode.

In the mixed paper mode, as described above, it is preferable that, in the case where the image information data value “Y” is intermediate, emphasis is placed on the reproducibility of a transparent color in an OHP sheet, and, in the case where the image information data value “Y” is large, emphasis is placed on glossiness in normal paper. In a practical use, generally, reduction of the reproducibility of a transparent color is more problematic than a change of glossiness. Therefore, in the mixed paper mode, the fixing conditions may be always corrected under the same conditions as those in the OHP sheet mode, irrespective of the image information data value “Y”.

Seventh Embodiment

Next, a seventh embodiment of the invention will be described.

In the seventh embodiment, in order to prevent the reduction of the fixing property in normal paper and that of reproducibility of a transparent color in an OHP sheet which are caused by thermal degradation of the silicone rubber layers 26 from occurring, the fixing conditions are corrected on the basis of the detected temperature “t” by the temperature sensor 33 in addition to the integrated energizing time and the image information data value “Y”.

The mechanical structure of the fixing device of the seventh embodiment is identical with that of the sixth embodiment shown in FIG. 22 , and the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 have a high heat conductivity which is not smaller than 1.2×10 −3 (cal/cm·sec·° C.) and not larger than 2.4×10 −3 (cal/cm·sec·° C.) in the same manner as those of the first embodiment. The fixing temperature before correction is 160° C. and the fixing speed is 150 mm/sec before correction.

In steps S 30 - 1 to S 30 - 4 of FIG. 30 , in the same manner as the sixth embodiment, the transfer sheet information is read, the image information is read, the image information data value “Y” is calculated, and the time information is read. Thereafter, the detected temperature “t” by the temperature sensor 33 is read into the controller 135 in step S 30 - 5 .

If it is judged in step S 30 - 6 that the current mode is not the mixed paper mode, and if it is judged in step S 30 - 8 that the transfer sheet 30 is normal paper, then correction of the fixing conditions for the normal paper mode, and the fixing operation are performed in steps S 30 - 9 to S 30 - 11 .

In the correction of the fixing conditions in step S 30 - 9 , as shown in steps S 31 - 1 to S 31 - 9 of FIG. 31 , the fixing conditions are corrected on the basis of the integrated energizing time ATM and the image information data value “Y” in the same manner as the correction of the fixing conditions for the normal paper mode in the sixth embodiment. In step S 31 - 10 , the difference (temperature difference Δt) between the detected temperature “t” and a set value (set temperature t SET ) of the fixing temperature is calculated. In steps S 31 - 11 to S 31 - 15 , the fixing speed is further corrected in accordance with the temperature difference Δt.

If it is judged in step S 31 - 11 that the temperature difference Δt is smaller than 2.5° C. (Δt=0 to −2.5° C.), then it is determined in step S 31 - 12 that the fixing speed is not corrected. If it is judged in step S 31 - 11 that the temperature difference Δt is not smaller than 2.5° C., then the control is advanced to step S 31 - 13 . If it is judged in step S 31 - 13 that the temperature difference Δt is smaller than 5° C. (Δt=−2.5 to −5° C.), then the fixing speed is further reduced by 3.5% in step S 31 - 14 . If it is judged in step S 31 - 13 that the temperature difference Δt is larger than 5° C., then the fixing speed is further reduced by 7% in step S 31 - 15 .

If it is judged in step S 30 - 8 that the transfer sheet 30 is an OHP sheet, then correction of the fixing conditions for the OHP sheet mode, and the fixing operation are performed in steps S 30 - 12 to S 30 - 14 .

In the correction of the fixing operation in step S 30 - 12 , as shown in steps S 32 - 1 to S 32 - 9 of FIG. 32 , the fixing conditions are corrected in accordance with the integrated energizing time ATM and the image information data value “Y” in the same manner as the correction of the fixing conditions for the OHP sheet mode in the sixth embodiment.

In step S 32 - 10 , the temperature difference Δt is calculated. In steps S 32 - 11 to S 32 - 15 , the fixing speed is corrected in accordance with the temperature difference Δt.

If it is judged in step S 32 - 11 that the temperature difference Δt is smaller than 2.5° C. (Δt=0 to −2.5° C.), then it is determined in step S 32 - 12 that the fixing speed is not corrected. If it is judged in step S 32 - 11 that the temperature difference Δt is not smaller than 2.5° C., then the control is advanced to step S 32 - 13 . If it is judged in step S 32 - 13 that the temperature difference Δt is smaller than 5° C. (Δt=−2.5 to −5° C.), then the fixing speed is reduced by 5% in step S 32 - 14 . If it is judged in step S 32 - 13 that the temperature difference Δt is larger than 5° C., then the fixing speed is reduced by 10% in step S 32 - 15 .

If it is judged in step S 30 - 6 that the current mode is the mixed paper mode, then the correction is advanced to step S 30 - 7 in which, in the same manner as the sixth embodiment, in the cases where the image information data value “Y” is small and where the image information data value “Y” is large, correction of the fixing conditions is performed under the same conditions as those in the normal paper mode, and, in the case where the image information data value “Y” is intermediate, correction of the fixing conditions is performed under the same conditions as those in the OHP sheet mode

As described above, in the seventh embodiment, the fixing speed is further corrected on the basis of the temperature difference Δt in any one of the normal paper mode, the OHP sheet mode, and the mixed paper mode, and the reduction amount of the fixing speed is set to be larger as the temperature difference Δt is larger. As the temperature difference Δt is larger, the actual surface temperatures of the silicone rubber layers 26 are lower. Therefore, when the fixing speed is decreased in accordance with the temperature difference Δt as described above, the reduction of the fixing property and that of reproducibility of a transparent color which are caused by reduction of hardness of the silicone rubber layers 26 due to the thermal degradation can be prevented more surely from occurring.

In also the seventh embodiment, if the image information data value “Y” is small in the mixed paper mode, the fixing conditions may be corrected under the same conditions as those in the OHP sheet mode. In the mixed paper mode, the fixing conditions may be always corrected under the same conditions as those in the OHP sheet mode, irrespective of the image information data value “Y”. Furthermore, the fixing temperature may be increased in accordance with the temperature difference Δt.

Eighth Embodiment

FIG. 33 shows a fixing device which is an eighth embodiment of the invention. The eighth embodiment is configured in the same manner as the first embodiment except that thermal degradation estimating means 231 for estimating the degree of thermal degradation of the silicone rubber layers 26 , and control means 232 for controlling the motors 17 and 28 and the heaters 27 are provided, and hence like components are denoted by the same reference numerals.

As shown in FIG. 34 , the thermal degradation estimating means 231 comprises fixing operation detecting means 235 for detecting whether the fixing device is performing the fixing operation or in a waiting state (in a non-fixing operation), and energization detecting means 236 for detecting whether the fixing device is in an energization state where a current is supplied from a power source (not shown) to the heaters 27 , or in a nonenergization state where no current is supplied to the heaters 27 .

The thermal degradation estimating means 231 further comprises temperature comparing means 238 , reading means 240 a for reading a measured value of a timer 239 a serving as energizing time measuring means, and energizing time integrating means 241 a for integrating the read value of the reading means 240 a . The thermal degradation estimating means 231 further comprises reading means 240 b for reading a measured value of a timer 239 b serving as waiting energizing time measuring means, and accumulating means 241 b (waiting energizing time integrating means) for integrating the read value of the reading means 240 b . The thermal degradation estimating means 231 further comprises storage means 242 for storing calculation results of the integrating means 241 a and 241 b , and degradation judging means 243 for judging the degree of thermal degradation of the silicone rubber layers 26 on the basis of values stored in the storage means 242 .

With reference to the flowchart shown in FIG. 35 , the operation of the eighth embodiment will be described. When the power source of the fixing device is turned on, the fixing operation detecting means 235 judges in step S 35 - 1 whether the fixing operation is being performed or not. If it is judged in step S 35 - 1 that the fixing operation is being performed, then the control is advanced to step S 35 - 2 .

In step S 35 - 2 , the energization detecting means 236 judges whether the heaters 27 are energized or not. If the heaters 27 are energized, then the control is advanced to step S 35 - 3 , and, if the heaters are not energized, then the control is returned to step S 35 - 1 .

In step S 35 - 3 , the temperature (detected temperature “t”) of the silicone rubber layer 26 of the pressure roller 15 which is detected by the temperature sensor 233 is supplied to the temperature comparing means 238 .

Next, the temperature comparing means 238 compares in step S 35 - 4 the detected temperature “t” with a reference temperature t std which is a preset reference value of the surface temperature (fixing temperature) of the silicone rubber layer 26 . If it is judged in step S 35 - 4 that the detected temperature “t” is not lower than the reference temperature “t std ”, then the timer 239 a is started in step S 35 - 5 .

If it is detected in step S 35 - 6 that the heaters 27 are not energized, then the timer 239 a is stopped in step S 35 - 7 . In step S 35 - 8 , the reading means 240 a reads out an energizing time TM1 which is measured by the timer 239 a . The energizing time integrating means 241 a calculates in step S 35 - 9 an integrated energizing time ATM1 which is an integrated value of the energizing time TM1. In step S 35 - 10 , the integrated energizing time ATM1 is stored into the storage means 242 .

By contrast, if the fixing operation detecting means 235 judges in step S 35 - 1 that the fixing device is not performing the fixing operation or is in the waiting state, then the control is advanced to step S 35 - 11 . If the energization detecting means 236 detects in step S 35 - 11 that the heaters 27 are being energized, then the timer 239 b is started in step S 35 - 12 . If the heaters are not energized in step S 35 - 11 , then the control is returned to step S 35 - 1 .

If it is detected in step S 35 - 13 that the heaters 27 are not energized, then the timer 239 b is stopped in step S 35 - 14 . In step S 35 - 15 , the reading means 240 b reads out a waiting energizing time TM2 measured by the timer 239 b . The integrating means 241 b calculates in step S 35 - 16 an integrated waiting energizing time ATM2 which is an integrated value of the waiting energizing time TM2. In step S 35 - 17 , the integrated waiting energizing time ATM2 is stored into the storage means 242 .

In step S 35 - 18 , the degradation judging means 243 judges the degree of thermal degradation of the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 , in accordance with the integrated energizing time ATM1 and the integrated waiting energizing time ATM2 which are stored in the storage means 242 .

Specifically, the degradation judging means 243 calculates an estimated degradation time DTM from following equation (2).

DTM=ATM1+ATM2×0.2  (2)

Next, the degradation judging means 243 calculates a degradation level DL indicating the degree of degradation of the silicone rubber layers 26 , from the estimated degradation time DTM and on the basis of FIG. 36 . In the embodiment, the degradation level DL is an integer of “0” to “4,” and indicates that, as the value is larger, the degradation is further advanced.

On the basis of the judgement result of the degradation judging means 243 , the control means 232 controls the motors 17 and 28 and the heaters 27 . Specifically, as shown in Table 25 listed below, the outputs of the heaters 27 are increased as the value of the degradation level DL is larger, so that the fixing temperature is set to be higher. As the thermal degradation is further advanced, the rotational speed of the motor 28 may be reduced so as to lower the transfer speed of the transfer sheet 30 . Alternatively, both the outputs of the heaters 27 , and the rotational speed of the motor 28 may be adjusted.

TABLE 25
Estimated degradation	Degradation	Corrected fixing
time DTM (Hr)	level DL	temperature (° C.)
0˜200	0	160
200˜800	1	161.5
800˜2000	2	163
2000˜5000	3	164.5
5000˜7000	4	166

In the eighth embodiment, since the degree of thermal degradation of the silicone rubber layers 26 is estimated as described above on the basis of the integrated energizing time ATM1 which is an integrated value of the energizing time TM1 of the heaters 27 during the fixing operation, the degree of thermal degradation can be estimated by a simple configuration. Further, in the eighth embodiment, since the degree of thermal degradation is estimated in consideration of also the integrated waiting energizing time ATM2 which is an integrated value of the waiting energizing time TM2 of the heaters 27 in the waiting state, the estimation can be performed in a highly accurate manner.

Ninth Embodiment

A fixing device of a ninth embodiment of the invention shown in FIG. 37 comprises set temperature switching means 245 for switchingly setting the set value of the fixing temperature (set temperature t set ) to one of three kinds of values, t set1 , t set2 , and t set3 . The set temperature switching means 245 may be configured so that the set temperature t set is manually input through an operation panel (not shown), or the set temperature is automatically switched over.

In the ninth embodiment, the thermal degradation estimating means 231 further comprises correction factor determining means 246 and correcting means 247 .

The other configuration of the ninth embodiment is identical with that of the eighth embodiment. Hence like components are denoted by the same reference numerals, and the description is omitted.

With reference to the flowchart shown in FIG. 38 , the operation of the ninth embodiment will be described. The fixing operation detecting means 235 judges in step S 38 - 1 whether the fixing operation is being performed or not. If it is judged in step S 38 - 1 that the fixing operation is being performed, then the control is advanced to step S 38 - 2 .

In step S 38 - 2 , the energization detecting means 236 judges whether the heaters 27 are energized or not. If the heaters are energized, then the control is advanced to step S 38 - 3 , and, if the heaters are not energized, then the control is returned to step S 38 - 1 .

In step S 38 - 3 , the detected temperature “t” which is detected by the temperature sensor 233 is supplied to the temperature comparing means 238 .

Next, the correction factor determining means 246 implements in step S 38 - 4 the processing shown in FIG. 39 to determine a correction factor “a”.

First, in step S 39 - 1 , it is judged whether the set temperature t set is equal to t set1 or not. If the set temperature is equal to t set1 , then the correction factor “a” is set to a 1 . If it is judged in step S 39 - 1 that the set temperature t set is not equal to t set1 , then the control is advanced to step S 39 - 3 . In step S 39 - 3 , it is judged whether the set temperature t set is equal to t set2 or not. If the set temperature is equal to t set2 , then the correction factor “a” is set to a 2 in step S 39 - 4 . If it is judged in step S 39 - 3 that the set temperature t set is not equal to t set2 , then the set temperature t set is equal to t set3 , and the correction factor “a” is set to a 3 in step S 39 - 5 .

Next, the temperature comparing means 238 compares in step S 38 - 5 of FIG. 38 the detected temperature “t” with the set temperature t set . If the detected temperature t is not lower than the set temperature t set , the timer 239 a is started in step S 38 - 6 .

If it is detected in step S 38 - 7 that the heaters 27 are not energized, then the timer 239 a is stopped in step S 38 - 8 . In step S 38 - 9 , the reading means 240 a reads out an energizing time TM1′ which is measured by the timer 239 a .

Next, in step S 38 - 10 , the energizing time TM1′ read by the reading means 240 a is multiplied with the correction factor “a” to calculate a corrected energizing time RTM1′. The corrected energizing time RTM1′ is shown by following expression (3).

RTM1′=a×TM1′  (3)

Next, the integrating means 241 a calculates in step S 38 - 11 an integrated corrected energizing time ARTM1′ which is an integrated value of the corrected energizing time RTM1′. In step S 38 - 12 , the integrated corrected energizing time ARTM1′ is stored into the storage means 242 .

By contrast, if the fixing operation detecting means 235 judges in step S 38 - 1 that the fixing device is not performing the fixing operation or is in the waiting state, then the processing of steps 38 - 13 to 38 - 19 is performed in the same manner as that of steps 35 - 11 to 35 - 17 (see FIG. 35 ) in the eighth embodiment.

In step S 38 - 20 , the degradation judging means 243 judges the degree of thermal degradation of the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 , on the basis of the integrated corrected energizing time ARTM1′ and the integrated waiting energizing time ATM2 which are stored in the storage means 242 . Specifically, the degradation judging means 243 calculates an estimated degradation time DTM from following expression (4).

DTM=ARTM1′+ATM2×0.2  (4)

Next, the degradation judging means 243 judges a degradation level DL from the estimated degradation time DTM and on the basis of FIG. 36 .

On the basis of Table 25 listed above, the control means 232 corrects the fixing temperature in accordance with the degradation level DL.

In the ninth embodiment, as described above, the degree of thermal degradation of the silicone rubber layers 26 is estimated on the basis of, in place of a simple integration of the energizing time during the fixing operation, the integrated corrected energizing time ARTM1′ which is an integrated value of the corrected energizing time RTM1′ that has been corrected by using the correction factor a corresponding to the set temperature t set . Therefore, the estimation can be performed in a more highly accurate manner.

Tenth Embodiment

In a tenth embodiment of the invention shown in FIG. 40 , the correction factor determining means 246 is connected to judging means 249 for judging the kind of the transfer sheet 30 , and determines the set temperature t set and the correction factor “a” on the basis of an input from the judging means 249 . In other words, the correction factor determining means 246 in the tenth embodiment is provided with also the function of the set temperature switching means 245 in the ninth embodiment.

The operation of the tenth embodiment is different from that of the ninth embodiment in that the determination of the correction factor “a” in step S 38 - 4 of FIG. 38 is performed on the basis of the flowchart shown in FIG. 41 .

In step S 41 - 1 of FIG. 41 , the correction factor determining means 246 judges whether the transfer sheet 30 is normal paper or not, on the basis of an input signal from the judging means 249 . If the transfer sheet 30 is normal paper, then the set temperature t set is set to t set1 in step S 41 - 2 , and the correction factor “a” is set to a 1 in step S 41 - 3 . By contrast, if it is judged in step S 41 - 1 that the transfer sheet 30 is not normal paper, then it is judged in step S 41 - 4 whether the transfer sheet 30 is thick paper or not. If it is judged in step S 41 - 4 that the transfer sheet 30 is thick paper, then the set temperature t set is set to t set2 in step S 41 - 5 , and then the correction factor “a” is set to a 2 in step S 41 - 6 . If it is judged in step S 41 - 4 that the transfer sheet 30 is not thick paper, then the set temperature t set is set to t set3 in step S 41 - 7 , and then the correction factor “a” is set to a 3 in step S 41 - 8 .

The other configuration and function of the tenth embodiment are identical with those of the ninth embodiment, and the description is omitted.

In the tenth embodiment, as described above, the degree of thermal degradation of the silicone rubber layers 26 is estimated on the basis of, in place of a simple integration of the energizing time during the fixing operation, the integrated corrected energizing time ARTM1′ which is an integrated value of the corrected energizing time RTM1′ that has been corrected by using the correction factor “a” corresponding to the kind of the transfer sheet 30 . Therefore, the estimation can be performed in a more highly accurate manner.

Eleventh Embodiment

In an eleventh embodiment of the invention shown in FIG. 42 , the temperature sensor 233 are connected to the temperature comparing means 238 and the correction factor determining means 246 .

The operation of the eleventh embodiment is different from that of the ninth embodiment in that the determination of the correction factor “a” in step S 38 - 4 of FIG. 38 is performed on the basis of the flowchart shown in FIG. 43 .

In step S 43 - 1 of FIG. 43 , the correction factor determining means 246 calculates the difference (temperature difference Δt) between the detected temperature t and the predetermined set temperature t set on the basis of following equation (5).

Δt=t set −t  (5)

Next, it is judged in step S 43 - 2 whether the temperature difference Δt is equal to or smaller than a first predetermined value t 1 or not. If it is judged in step S 43 - 2 that the temperature difference Δt is equal to or smaller than the first predetermined value t 1 , then the correction factor “a” is set to a 1 in step S 43 - 3 . If it is judged in step S 43 - 2 that the temperature difference Δt is not equal to nor smaller than the first predetermined value t 1 , then it is judged in step S 43 - 4 whether the temperature difference Δt is equal to or smaller than a second predetermined value t 2 or not. If it is judged in step S 43 - 4 that the temperature difference Δt is equal to or smaller than the second predetermined value t 2 , then the correction factor “a” is set to a 2 in step S 43 - 5 . If the temperature difference Δt is not equal to nor smaller than the predetermined value t 2 , then the correction factor “a” is set to a 3 in step S 43 - 6 .

The other configuration and function of the eleventh embodiment are identical with those of the ninth embodiment, and the description is omitted.

In the eleventh embodiment, as described above, the degree of thermal degradation of the silicone rubber layers 26 is estimated on the basis of, in place of a simple integration of the energizing time during the fixing operation, the integrated corrected energizing time ARTM1′ which is an integrated value of the corrected energizing time RTM1′ that has been corrected by using the correction factor “a” corresponding to the temperature difference Δt between the set temperature t set and the detected temperature “t”. Therefore, the estimation can be performed in a more highly accurate manner.

Twelfth Embodiment

In a twelfth embodiment of the invention shown in FIG. 44 , in the same manner as the ninth embodiment, the thermal degradation estimating means 231 comprises the correction factor determining means 246 , the temperature comparing means 238 , the timer 239 a , the reading means 240 a , the correcting means 247 , and the integrating means 241 a which are used for calculating the integrated corrected energizing time ARTM1′. The thermal degradation estimating means 231 further comprises a timer 239 b , the reading means 240 b , and the integrating means 241 b which are used for calculating the integrated waiting energizing time ATM2. The thermal degradation estimating means 231 further comprises, a timer 239 c , reading means 240 c , and integrating means 241 c which are provided for calculating the energizing time TM1″ and an integrated energizing time ATM1″ that is an integrated value of the energizing time TM1″, irrespective of the detected temperature “t”.

With reference to the flowchart shown in FIG. 45 , the operation of the twelfth embodiment will be described. If the fixing operation detecting means 235 judges in step S 45 - 1 that the fixing operation is being performed, then the control is advanced to step S 45 - 2 . Then, the energization detecting means 236 judges whether the heaters 27 are energized or not. If the heaters 27 are energized, then the control is advanced to step S 45 - 3 , and, if the heaters are not energized, then the control is returned to step S 45 - 1 .

In step S 45 - 3 , the timer 239 c is started, and, in step S 45 - 4 , the detected temperature “t” is supplied from the temperature sensor 233 to the temperature comparing means 238 .

Next, the correction factor determining means 246 determines in step S 45 - 5 the correction factor “a” in the same method as that of the ninth embodiment (see FIG. 38 ). Alternatively, the correction factor “a” may be determined in the same method as that of the tenth embodiment shown in FIG. 41 or that of the eleventh embodiment shown in FIG. 43 .

If the temperature comparing means 238 judges in step S 45 - 6 that the detected temperature t is not lower than the set temperature t set set by the set temperature switching means 245 , then the timer 239 a is started in step S 45 - 7 .

If it is detected in step S 45 - 8 that the heaters 27 are not energized, then the timers 239 a and 239 c are stopped in steps S 45 - 9 and S 45 - 10 .

In step S 45 - 11 , the reading means 240 a reads out the energizing time TM1′ which is measured by the timer 239 a , and, in step S 45 - 12 , the reading means 240 c reads out the energizing time TM1″ which is measured by the timer 239 c .

In step S 45 - 13 , the correcting means 247 corrects the energizing time TM1′ by expression (3) listed above to calculate the corrected energizing time RTM1′.

In step S 45 - 14 , the accumulating means 241 c calculates the integrated energizing time ATM1″ which is an integrated value of the energizing time TM1″. As seen from steps S 45 - 2 , S 45 - 3 , S 45 - 7 , S 45 - 8 , and S 45 - 9 , the integrated energizing time ATM1″ is obtained by simply integrating the energizing time during the fixing operation irrespective of the detected temperature t and the set temperature t set . In step S 45 - 15 , the integrating means 241 a calculates the integrated corrected energizing time ARTM1′ which is an integrated value of the corrected energizing time RTM1′.

In step S 45 - 16 , the integrated energizing time ATM1″ is stored into the storage means 242 , and, in step S 45 - 17 , the integrated corrected energizing time ARTM1′ is stored into the storage means 242 .

By contrast, if the fixing operation detecting means 235 judges in step S 45 - 1 that the fixing device is not performing the fixing operation, then the processing of steps S 45 - 18 to S 45 - 24 is performed in the same manner as that in the eighth embodiment so that the integrated waiting energizing time ATM2 is calculated and the calculated time is stored into the storage means 242 .

In step S 45 - 25 , the degradation judging means 243 judges the degree of thermal degradation of the silicone rubber layers 26 of the fixing roller 14 and the pressure roller 15 , on the basis of the integrated energizing time ATM1″, the integrated corrected energizing time ARTM1′, and the integrated waiting energizing time ATM2 which are stored in the storage means 242 . Specifically, the degradation judging means 243 first calculates the estimated degradation time DTM from following expression (6).

DTM=(ATM1″-ATM2)×0.5+ARTM1′+ATM2×0.2  (6)

Next, the degradation judging means 243 judges the degradation level DL from the estimated degradation time DTM and on the basis of FIG. 36 . The control means 232 corrects the fixing temperature on the basis of Table 25 listed above and in accordance with the degradation level DL. In the twelfth embodiment, since the degree of thermal degradation is estimated on the basis of the three kinds of integrated energizing times as described above, the estimation can be performed in a highly accurate manner.

Although the present invention has been fully described by way of the examples with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those who skilled in the art. Therefore, unless such changes and modifications otherwise depart from the spirit and scope of the present invention, they should be construed as being therein.

Claims

1. A fixing device comprising:first and second fixing members each of which has an elastic member and which are abutted against each other so that said elastic members contact with each other; a heater which heats at least one of said first and second fixing members; and a controller which corrects fixing conditions in accordance with an integrated energizing time that is an integrated value of an energizing time of said heater, wherein a sheet carrying an image of a developer is passed between said first and second fixing members, whereby the developer is heated and fused under a pressure exerted by said first and second fixing members, to be fixed onto the sheet.

2. A fixing device as in claim 1, wherein said fixing device further comprises a device which maintains constant a length of a contact portion between said first and second fixing members in a direction along which the sheet is passed.

3. A fixing device as in claim 1, wherein the fixing conditions include a fixing speed.

4. A fixing device as in claim 3, wherein said controller decreases the fixing speed in accordance with the integrated energizing time.

5. A fixing device as in claim 4, wherein said controller further decreases the fixing speed in a stepwise manner as the integrated energizing time is longer.

6. A fixing device as in claim 1, wherein the fixing conditions include a fixing temperature.

7. A fixing device as in claim 6, wherein said controller increases the fixing temperature in accordance with the integrated energizing time.

8. A fixing device as in claim 7, wherein said controller further increases the fixing temperature in a stepwise manner as the integrated energizing time is longer.

9. A fixing device as in claim 8, wherein said fixing device further comprises means for switchingly setting the fixing temperature to one of plural set values, andsaid controller increases the fixing temperature at a shorter interval with respect to the integrated energizing time as the fixing temperature set by said setting means is higher.

10. A fixing device as in claim 8, wherein said fixing device further comprises means for switchingly setting the fixing temperature to one of plural set values, andsaid controller increases the fixing temperature at a larger step width as the fixing temperature set by said setting means is higher.

11. A fixing device as in claim 1, wherein said fixing device further comprises a detector which detects a surface temperature of said elastic member of at least one of said first and second fixing members, andsaid controller corrects at least one of a fixing speed and a fixing temperature in accordance with the surface temperature of said elastic member which is detected by said detector.

12. A fixing device as in claim 1, wherein said fixing device further comprises a device which adjusts an abutting force between said first and second fixing members, andthe fixing conditions include the abutting force between said first and second fixing members.

13. A fixing device as in claim 12, wherein said abutting force adjusting device includes an urging member which urges at least one of said first and second fixing members in a direction along which the abutting force is increased, andsaid controller adjusts an urging force of said urging member, thereby correcting the abutting force between said first and second fixing members.

14. A fixing device as in claim 12, wherein said fixing device further comprises a device which maintains constant a length of a contact portion between said first and second fixing members in a direction along which the sheet is passed.

15. A fixing device as in claim 12, wherein said controller decreases the abutting force in accordance with the integrated energizing time.

16. A fixing device as in claim 1, wherein each of said first and second fixing members has a roller shape.

17. A fixing device comprising:first and second fixing members each of which has an elastic member and which are abutted against each other so that said elastic members contact with each other; a heater which heats at least one of said first and second fixing members; and a controller which corrects fixing conditions in accordance with an integrated energizing time that is an integrated value of an energizing time of said heater, and information of an image to be fixed, wherein a sheet carrying an image of a developer is passed between said first and second fixing members, whereby the developer is heated and fused under a pressure exerted by said first and second fixing members, to be fixed onto the sheet.

18. A fixing device as in claim 17, wherein the fixing conditions include at least one of a fixing speed and a fixing temperature.

19. A fixing device as in claim 17, wherein said fixing device further comprises a device which maintains constant a length of a contact portion between said first and second fixing members in a direction along which the sheet is passed.

20. A fixing device as in claim 17, wherein the image information includes a density and an area of the image to be fixed.

21. A fixing device as in claim 20, wherein said controller corrects the fixing conditions so that, in the case where the sheet on which the image is to be fixed is normal paper, a quantity of heat supplied to the sheet is increased in a stepwise manner as the integrated energizing time is longer, and, the quantity of heat supplied to the sheet is increased as an area of a high-density portion of the image to be fixed is larger.

22. A fixing device as in claim 20, wherein said controller corrects the fixing conditions so that, in the case where a sheet on which the image is to be fixed is an OHP sheet, a quantity of heat supplied to the sheet is increased in a stepwise manner as the integrated energizing time is longer, and, the quantity of heat supplied to the sheet is increased as an area of a half-tone portion of the image to be fixed is larger.

23. A fixing device as in claim 20, wherein, in the case where a sheet on which the image is to be fixed is normal paper, said controller corrects the fixing conditions so that, a quantity of heat supplied to the sheet is increased in a stepwise manner as the integrated energizing time is longer, and, the quantity of heat supplied to the sheet is increased as an area of a high-density portion of the image to be fixed is larger,wherein, in the case where the sheet on which the image is to be fixed is an OHP sheet, said controller corrects the fixing conditions so that, the quantity of heat supplied to the sheet is increased in a stepwise manner as the integrated energizing time is longer, and, the quantity of heat supplied to the sheet is increased as an area of a half-tone portion of the image to be fixed is larger, and wherein, in the case where the sheets on which the image is to be fixed include both normal paper and an OHP sheet, said controller corrects the fixing conditions under same conditions as the conditions of normal paper when the area of a high-density portion of the image to be fixed is large, and corrects the fixing conditions under same conditions as the conditions of an OHP sheet when the area of a half-tone portion of the image to be fixed is large.

24. A fixing device as in claim 20, wherein, in the case where a sheet on which the image is to be fixed is normal paper, said controller corrects the fixing conditions so that a quantity of heat supplied to the sheet is increased in a stepwise manner as the integrated energizing time is longer, and, the quantity of heat supplied to the sheet is increased as an area of a high-density portion of the image to be fixed is larger,wherein, in the case where the sheet on which the image is to be fixed is an OHP sheet, said controller corrects the fixing conditions so that the quantity of heat supplied to the sheet is increased in a stepwise manner as the integrated energizing time is longer, and, the quantity of heat supplied to the sheet is increased as an area of a half-tone portion of the image to be fixed is larger, and, wherein, in the case where the sheets on which the image is to be fixed include both normal paper and an OHP sheet, said controller corrects the fixing conditions under same conditions as the conditions of the OHP sheet.

25. A fixing device as in claim 17, wherein said controller corrects the fixing conditions in accordance with a surface temperature of said elastic member of at least one of said first and second fixing members in addition to the integrated energizing time and the information of the image to be fixed.

26. A fixing device as in claim 17, wherein each of said first and second fixing members has a roller shape.

27. A fixing device comprising:first and second fixing members each of which has an elastic member and which are abutted against each other so that said elastic members contact with each other; a heater which heats at least one of said first and second fixing members; and estimating means for estimating a degree of thermal degradation of said elastic members on the basis of an integrated energizing time that is an integrated value of an energizing time of said heater, wherein a sheet carrying an image of a developer is passed between said first and second fixing members, whereby the developer is heated and fused under a pressure exerted by said first and second fixing members, to be fixed onto the sheet.

28. A fixing device as in claim 27, wherein said fixing device further comprises a controller which controls fixing conditions in accordance with the degree of the thermal degradation of said elastic members estimated by said estimating means.

29. A fixing device as in claim 27, wherein said estimating means includes time measuring means for measuring the energizing time of said heater, and integrating means for integrating the energizing time measured by said time measuring means, to obtain the integrated value, andwherein said time measuring means starts time measurement when a surface temperature of said elastic member of at least one of said first and second fixing members becomes not lower than a predetermined reference temperature.

30. A fixing device as in claim 27, wherein said fixing device further comprises means for switchingly setting a fixing temperature to one of plural set values,wherein said estimating means includes time measuring means for measuring the energizing time of said heater, and integrating means for multiplying the energizing time measured by said measuring means with a correction factor corresponding to the fixing temperature set by setting means and integrating the products, to obtain the integrated value, and wherein said time measuring means starts time measurement when a surface temperature of said elastic member of at least one of said first and second fixing members becomes not lower than the fixing temperature set by said setting means.

31. A fixing device as in claim 27, wherein said estimating means includes time measuring means for measuring the energizing time of said heater, and integrating means for multiplying the energizing time measured by said time measuring means with a correction factor corresponding to a difference between a surface temperature of said elastic member of at least one of said first and second fixing members and a predetermined reference temperature and integrating the products, to obtain the integrated value, andwherein said time measuring means starts time measurement when a surface temperature of said elastic member of at least one of said first and second fixing members becomes not lower than the predetermined reference temperature.

32. A fixing device as in claim 27, wherein said estimating means includes first and second time measuring means for measuring the energizing time of said heater, and integrating means for integrating first and second energizing times which are measured by said first and second time measuring means, to obtain first and second integrated values,wherein the integrated energizing time includes the first and second integrated values, wherein said first time measuring means starts time measurement when energization to said heater is started, and wherein said second time measuring means starts time measurement when a surface temperature of said elastic member of at least one of said first and second fixing members becomes not lower than a predetermined reference temperature.

33. A fixing device as in claim 32, wherein said integrating means multiplies the first energizing time with a predetermined correction factor and integrates the products, to obtain the first integrated value.

34. A fixing device as in claim 27, wherein the integrated energizing time is an integrated value of the energizing time of said heater during a fixing operation, andwherein said estimating means estimates the degree of thermal degradation of said elastic members on the basis of an integrated value of the energizing time of said heater in a waiting state in which the fixing operation is not performed, in addition to the integrated energizing time.

35. A fixing device as in claim 27, wherein each of said first and second fixing members has a roller shape.

36. A fixing device as in claim 27, wherein said fixing device further comprises a device which maintains constant a length of a contact portion between said first and second fixing members in a direction along which the sheet is passed.