Image forming apparatus with heating members having standby mode and low power mode

An image forming apparatus including an image forming unit that forms an image on a recording material, a rotating member that heats the image on the recording material by a nip portion thereof, an executing unit capable of executing a processing of rotating the rotating member at standby, a measuring unit that measures a standby time period, and a changing unit that changes a time interval of executing the processing in accordance with the standby time period.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus using an electrophotography system, particularly relates to an image forming apparatus of a copier, a printer, a facsimile or the like.

2. Description of the Related Art

As an image forming apparatus adopting an electrostatic recording system or an electrophotography recording system, a copier, a printer, a facsimile apparatus or the like has been known. As an image forming apparatus of this kind, there is widely used a so-called roller type fixing apparatus for fixing a toner image onto a sheet by a pair of rollers brought into press contact with each other. According to the roller type fixing type apparatus, by bringing a fixing roller and a pressing roller of silicone rubber or the like into press contact with each other, a nip in a planar shape is formed at a contact face of the two rollers and fixing is executed by applying pressure and heat to the sheet at the nip portion.

At the nip portion, a rubber structure of the two rollers is always subjected to compressive strain. Therefore, when a stationary state of the fixing roller is continued for a long period of time, bonding of the rubber structure may be destructed by exceeding a plastic limit and the strain may not be recovered to nullify. Such a state is referred to as a compression set (compressive permanent set). The rubber roller causing the compression set is bent and therefore, the rubber roller cannot form an image or carry the sheet correctly in the fixing apparatus, and jamming, color shift, sheet skewing or the like is brought about. Hence, there has been proposed a method of preventing the compression set by shifting the nip position by rotating the rollers at constant time intervals in a standby mode in which the fixing roller is stationary (Japanese Patent Unexamined publication No. HEI 4-74708).

Meanwhile, in recent years, as is seen also in international standards of energy star and the like, reduction and the efficient formation of power consumption of a power consuming apparatus has strongly been promoted. Also an image forming apparatus is not exceptional and there has been devised an apparatus of adopting a measure of reducing power consumption of shifting from a standby mode to an energy saving mode of reducing power consumption when the image forming apparatus is not operated for a constant period of time in a standby mode in order to reduce power consumption at standby.

For example, when the above-described nip position change control executed at constant time intervals is adopted in such an energy saving mode, since an initial temperature of the fixing roller in shifting to the energy saving mode is not taken into consideration, there is a case in which a frequency of executing nip position change operation becomes excessively large or a case in which the frequency becomes excessively small. That is, there is a possibility that the nip position change operation is not carried out efficiently and properly.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an image forming apparatus capable of efficiently executing a processing of rotating a rotating member at standby.

It is other object of the invention to provide an image forming apparatus capable of prolonging service life of parts for rotating a rotating member at standby.

It is other object of the invention to provide an image forming apparatus capable of making restraint of deterioration in a rotating member and a reduction in power consumption compatible with each other by a simple constitution.

A further object of the invention will become apparent by reading the following detailed description in reference to the attached drawings.

According to a first aspect of the invention, there is provided an image forming apparatus comprising:

an image forming unit that forms an image on a recording material;

a rotating member that heats the image on the recording material by a nip portion thereof;

an executing unit capable of executing a processing of rotating the rotating member at standby;

a measuring unit that measures a standby time period; and

a changing unit that changes a time interval of executing the processing in accordance with the standby time period.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a simple explanation will be given of an outline of a nip position change control in an image forming apparatus according to an embodiment of the invention.

FIG. 1shows a relationship between a temperature of a fixing roller (abscissa) and a time period of generating a compression set (ordinate). As is known from the graph, the larger the heat amount provided to the fixing roller (that is, the higher the temperature of the fixing roller), the shorter the time period of changing a compressive strain of the fixing roller to a compression set.

A temperature of the fixing roller in an energy saving mode as a low power mode (control target temperature) is set to a value lower than that in a standby mode and therefore, also a time period of generating the compression set in the energy saving mode becomes longer than that in the standby mode. Therefore, when considered simply, an interval (executing time interval) of the nip position change operation (also referred to as “compression set preventing rotational operation”) in the energy saving mode may simply be set to be longer than that in the standby mode.

However, as a result of investigation by the inventor, it has been found that the following problem is posed by such a simple control. When the standby mode is switched to the energy saving mode, a control temperature of the fixing roller is changed from a standby mode setting temperature (first temperature) to an energy saving mode setting temperature (second temperature). However, at this occasion, an actual temperature of the fixing roller is not immediately lowered to the second temperature but gradually changed from the first temperature to the second temperature.

FIG. 2shows a transition of a state of the image forming apparatus (abscissa) and a detected temperature of the fixing roller (ordinate; bold line) and a control target temperature of the fixing roller is shown by a broken line of the abscissa.

It is known from the graph that a time period T is required until the temperature of the fixing roller is brought into a substantially equilibrium state at the second temperature. Therefore, when a nip position change interval is immediately switched from a value thereof for the first temperature in the standby mode to a value thereof for the second temperature in the energy saving mode simultaneously with switching from the standby mode to the energy saving mode, there is a concern of generating the compression set during a section of time until elapse of the time period T from time of switching the mode.

Hence, according to this embodiment, the interval of switching the nip position change operation is not switched simultaneously with switching from the standby mode to the energy saving mode but the interval is changed in consideration of a transition from the first temperature to the second temperature of the fixing roller. Thereby, even during the section of time until the temperature of the fixing roller reaches the second temperature after switching from the standby mode to the energy saving mode, the nip position change operation can be executed at a pertinent timing capable of restraining the compression set. Further, since a frequency of the nip position change operation can be optimized, the restraint of the compression set and the reduction in the power consumption at standby can be made to be compatible with each other. Further, a load on parts constituting a drive power source for driving the fixing roller can be alleviated.

The interval of executing the nip position change operation is changed in accordance with an elapsed time period after switching from the standby mode to the energy saving mode. Specifically, there is adopted a method of changing the interval of executing the nip position change operation to be prolonged in steps in accordance with the elapsed time period. Therefore, an influence of noise becomes difficult to be effected than in a constitution of changing the executing interval in accordance with the detected temperature (analog value) of the fixing roller and the processing can be simplified since there is only needed a control of a timer of measuring the elapsed time period. Further, a correlation between the elapsed time period and the change of the roller temperature is previously provided by an experiment or the like.

FIG. 3shows an example of the nip position change control according to the embodiment of the invention. The drawing illustrates time charts showing the nip position change control when the mode is successively switched such that standby mode→print mode→standby mode→energy saving mode.

(1) In the standby mode, by controlling an amount of current through a fixing heater105, mentioned later, the temperature of the fixing roller is maintained at the standby mode setting temperature (first temperature) capable of forming an image (capable of executing a fixing processing) At this occasion, a drive power source for driving a fixing motor as driving means is normally made to be ON to bring about a state of capable of starting the fixing motor at any time. Further, under control by a CPU501as an executing unit, the fixing motor drives the fixing roller to execute the nip position change operation at an interval of 5 minutes.

(2) In the print mode, since the fixing roller is normally rotated and therefore, the nip position change operation is not needed.

(3) Printing is finished, the print mode is switched to the standby mode by the CPU501, and the fixing motor is stopped to stop rotating the fixing roller. Further, similar to (1), the nip position change operation is executed at the interval of 5 minutes.

(4) When a predetermined time period has elapsed while staying in the standby mode, the mode is switched to the energy saving mode automatically by the CPU501. In the energy saving mode, the drive power source for the fixing motor is shut off to bring about the state in which a more time period is required for starting the fixing motor than in the standby mode. Further, a timer is started and an elapsed time period from a time point of switching the mode is started to measure. When the elapsed time period is less than 30 minutes, the drive power source for the fixing motor is started up at an executing interval of 5 minutes equal to that in the case of the standby mode, the nip position change operation is successively executed, and after finishing the nip position change operation, the drive power source is shut off to bring about the state of the energy saving mode again. Further, when 30 minutes or more has elapsed, the interval is changed to 10 minutes and thereafter, the nip position change operation is executed at the interval of 10 minutes.

Now, a detailed explanation will be given of a specific constitution of the image forming apparatus executing the above-described control.

<Constitution of Image Forming Apparatus>

FIG. 4shows a constitution of the image forming apparatus according to the embodiment of the invention. The image forming apparatus is a copier of an electrophotography recording system and is constituted by a document reading portion1R for reading an image of a document and an image outputting portion1P for outputting an image to a sheet (recording material).

The image outputting portion1P is constituted by an image forming unit10(in which four stations a, b, c, d are installed in parallel and constitutions thereof are the same), a sheet feeding unit20, an intermediate transferring unit30, a fixing unit40and a control unit (FIG. 6) in gross classification.

The image forming unit10is constructed by a constitution described below. Photosensitive drums11a,11b,11c, and11das image bearing members are axially supported at centers thereof and driven to rotate in arrow mark directions. Surroundings of the photosensitive drums11athrough11dare arranged with primary chargers12a,12b,12c, and12d, optical systems13a,13b,13c, and13d, reflecting mirrors16a,16b,16cand16dand developing devices14a,14b,14c, and14dalong rotational directions of the photosensitive drums.

The sheet feeding unit20comprises cassettes21a,21bfor containing a sheet P, a hand setting tray27, pick up rollers22a,22b,and26of feeding the sheets P sheet by sheet from inside of the cassettes or the hand setting tray, sheet feed roller pairs23and sheet feed guides24for conveying the sheets P fed out from the respective pickup rollers to register rollers, and register rollers25a,25bfor feeding the sheets P to a secondary transferring region Te in accordance with a timing of forming the image in the image forming unit.

The intermediate transferring unit30will be explained in details. An intermediate transferring belt31is an intermediate transferring member produced from a material of, for example, PET [polyethyleneterephthalate] or PVdF [polyvinyl fluoride].

The intermediate transferring belt31is wound around a drive roller32for transmitting a drive force in the direction of arrow B to the intermediate transferring belt31, a tension roller33for exerting a pertinent tension to the intermediate transferring belt31by being urged by a spring (not illustrated), and a driven roller34opposed to the secondary transferring region Te by interposing the belt. Among them, a primary transferring plane A is formed between the drive roller32and the tension roller33. The drive roller32prevents slip relative to the belt by coating rubber (urethane or chloroprene) having a thickness of several mm on a surface of a metal roller. The drive roller32is driven to rotate by a pulse motor (not illustrated).

Primary transferring regions Ta through Td constituting portions of bringing the respective photosensitive drums11athrough11dand the intermediate transferring belt31into contact with each other are arranged with primary transferring blades (primary transferring chargers)35athrough35don a rear side of the intermediate transferring belt31.

A secondary transferring roller36is arranged to be opposed to the driven roller34to form the secondary transferring region Te by a nip between the intermediate transferring belt31and the secondary transferring roller36. The secondary transferring roller36is pressed to the intermediate transferring belt31by a pertinent pressure.

A cleaning device50for cleaning an image forming face of the intermediate transferring belt31is arranged on the intermediate transferring belt and on a downstream side of the secondary transferring region Te. The cleaning device50comprises a cleaner blade51(polyurethane rubber or the like is used as a material therefor) and a waste toner box52for containing waste toner.

The fixing unit (fixing means)40is provided with a fixing roller (rotating member)41aand a pressing roller (second rotating member)41bconstituting a pair of rotating members brought into press contact with each other. Further, the fixing unit40is provided with a guide43for guiding the sheet P to the nip portion of the roller pair, inner sheet discharge rollers44, outer sheet discharge rollers45for guiding the sheet P discharged from the roller pair further to a tray48outside of the apparatus.

The control unit comprises a control board for controlling operation of mechanisms in the respective units and a motor drive board.

Further, a detailed explanation will be given of data of the fixing unit40and the control unit in reference to other drawings.

Next, image forming operation will be explained by taking an example of a case of forming an image on the sheet P contained in the cassette21a.

When an image forming operation start signal is generated, first, the sheet P is fed out sheet by sheet from the cassette21aby the pickup roller22a.Further, the sheet P is conveyed to the register rollers25a,25bby the sheet feed roller pairs23by being guided between the sheet feed guides24. At this occasion, the register rollers are brought into a stationary state and a front end of the sheet is butted to the nip portion. Thereafter, the register rollers25aand25bstart rotating in accordance with a timing of starting to form an image by the image forming unit10. The rotation start timing is set such that the sheet P and the toner image primarily transferred on the intermediate transferring belt31by the image forming unit10precisely coincide with each other at the secondary transferring region Te.

Meanwhile, at the image forming unit10, when an image forming operation start signal is generated, the primary chargers12athrough12dapply charge to the photosensitive drums11athrough11dto charge surfaces thereof uniformly. Successively, by exposing light ray (for example, laser beam) modulated by the optical system13athrough13din accordance with record image signals on the photosensitive drums11athrough11d,an electrostatic latent image is formed there. Further, the developing devices14athrough14drespectively containing 4 colors of developers (toners) of yellow, cyan, magenta, black visualize the electrostatic latent image. The visualized visible image is transferred onto the intermediate transferring member at the image transferring regions Ta, Tb, Tc, and Td. On the downstream side of the image transferring regions Ta through Td, the cleaning device15a,15b,15c,and15dscrape off the toners remaining on the photosensitive drums11athrough11dwithout being transferred onto the intermediate transferring member to clean the surfaces of the drums. By the above-described process, the image is successively formed by the respective toners.

Further, the toner image formed on the photosensitive drum11ddisposed on the uppermost side in the rotational direction of the intermediate transferring belt31is primarily transferred onto the intermediate transferring belt31at the primary transferring region Td by the primary transferring blade35dapplied with a high voltage. The primarily transferred toner image is carried to the successive primary transferring region Tc. At the station c, the image is formed by being delayed from the station d by a time period of carrying the toner image. Therefore, at the primarily transferring region Tc, the successive toner image is transferred on the previously transferred toner image by matching a resist thereof. Similar steps are repeated as follows, as a result, 4 colors of the toner images are overlapped on the intermediate transferring belt31.

A high voltage is applied on the secondary transferring roller36in accordance with a timing of advancing the sheet P to the secondary transferring region Te. Further, 4 colors of the toner images formed on the intermediate transferring belt are transferred onto the surface of the sheet P. Thereafter, the sheet P is guided to the nip portion of the fixing unit40by the carry guide43. Further, heat and pressure are applied to the nip portion of the fixing roller41aand the pressing roller41band the toner image is fixed onto the surface of the sheet. Thereafter, the sheet P is conveyed by the inner and outer sheet discharging rollers44,45to discharge outside of the apparatus.

FIG. 5shows a section of the roller pair of the fixing unit40.

The fixing roller41ais a rubber roller and comprises a core metal103in a cylindrical shape and a rubber layer wrapped around the core metal103. Further, also the pressing roller41bis a rubber roller and comprises a core metal104in a cylindrical shape and a rubber layer as an elastic layer wrapped around the core metal104. Silicone rubber or the like is preferable for a material of the rubber layer and a layer thickness thereof is formed to be comparatively thick. As a material of the core metals103and104, a material having an excellent heat conductivity of stainless steel, aluminum, copper or the like is preferable. The fixing roller and the pressing roller are brought into a state of being brought into press contact with each other by a predetermined pressure to form the fixing nip portion. Further, there is constructed a constitution that the pressing roller brought into press contact with the fixing roller is driven to rotate by driving to rotate the fixing roller by a fixing drive motor. That is, the pressing roller is also rotated along with the fixing roller by the nip position change operation, mentioned later.

The fixing heater105constituting a first heating source is provided at an inner portion of the fixing roller41a.A halogen heater can preferably be adopted for the fixing heater105. The fixing heater105generates heat by conducting electricity to warm the core metal103. The core metal103serves to warm the fixing roller41aby transferring heat supplied by the fixing heater105to the fixing roller41a.

A temperature holding heater106constituting a second heating source is provided at an inner portion of the pressing roller41b.That is, the fixing unit40according to the embodiment includes the plurality of heating sources (fixing heater105, temperature holding heater106) for heating the rotating members (fixing roller41a,pressing roller41b). It is preferable to adopt a halogen heater also for the temperature holding heater106.

A surrounding of the fixing roller41ais arranged with a first surface temperature sensor107and a second surface temperature sensor108. The first surface temperature sensor107measures a surface temperature at a center portion in an axial direction of the fixing roller41a.

The control unit maintains the temperature of the fixing roller41ato be at a predetermined control temperature by controlling to switch ON/OFF the fixing heater105based on output of the first surface temperature sensor107.

The second surface temperature sensor108measures a surface temperature of an end portion in the axial direction of the fixing roller41a.

The second surface temperature sensor108serves to prevent the fixing unit40from being failed beforehand by excessively heating the fixing roller41awhen the surface temperature cannot correctly be measured by a failure of the first surface temperature sensor107.

FIG. 6is a block diagram showing a constitution of the control unit for controlling the image forming apparatus of the embodiment. The control unit is provided with the CPU (central processing unit)501, an image reader control unit502, an image signal control unit503, a printer control unit504, a ROM (read only memory)505, a RAM (random access memory)506, an operation panel control unit507, an A/D converter508, a power source switch509.

The CPU501controls the document reading portion1R via the image reader control unit502by executing a program stored in the ROM505. The image signal control unit503stores image data of a document read by the document reading portion1R, or image data inputted via a network and outputs print data to the printer control unit504. The CPU501controls an operation panel (not illustrated) via the operation panel control unit507. The CPU501detects the surface temperature of the fixing roller41aby converting an analog output of the first and the second surface temperature sensor107and108into digital data by the A/D converter508. Further, the CPU501pertinently controls to supply/cut power for driving respective constituent elements including the fixing unit40by starting up/shutting off the fixing motor drive power source by the power source switch509.

FIG. 7shows allocation of areas of the ROM505and RAM506.

A storage area601of the ROM505includes a program area603stored with a program, a static parameter area604stored with a static parameter necessary for executing the program, an area605stored with an interval table (TblRotIntvl) registered with a plurality of nip position change (compression set preventing rotation) intervals, and an area606stored with switch time period (threshold data) of the nip position change interval (TblRotChng). According to the embodiment, as an example of the switch time period TblRotChng, a value of “30 minutes” is stored.

A storage area602of the RAM506includes a stack area607and a variable area608necessary for executing a program, an area609for an interval timer (TimeRotIntvl) used for counting an elapsed time period from executing the nip position change operation at a preceding time, an area610for an interval switch timer (TimeRotChng) for counting an elapsed time period after switching the standby mode to the energy saving mode, and an area611for counting up time stored with a value of a nip position change interval (TimeUpRotIntvl). The interval timer TimeRotIntvl is a timer for counting a timing of executing the nip position change operation and the interval switch timer TimeRotChng is a timer for counting a value of the nip position change interval TimeUpRotIntvl, that is, a timing of changing an interval of executing the nip position change operation.

<Temperature Control of Fixing Unit>

FIG. 8is a flowchart showing a temperature control algorithm of the fixing unit. The processing is executed by the CPU501(program).

When the power source of the image forming apparatus is switched on, an initial value of “190° C.” is stored to a control target temperature storing variable Tref (step701). When a state (mode) of the image forming apparatus is changed (step702), a control target temperature in accordance with the mode is read from a control target temperature table and the value of the control target temperature is stored to the variable Tref (step703).

FIG. 9shows an example of the control target temperature table. The table defines relations between modes of the image forming apparatus and control target temperatures (set temperatures). The image forming apparatus of the embodiment includes a warm up mode of a state of operating to prepare printing, a standby mode of a printable state, a print mode of a state of executing printing, an energy saving mode of a state of minimizing power consumption, and an emergency stop mode.

The print mode includes a print start mode, a print2mode, a print3mode and a print4mode which are shifted in steps in accordance with an elapsed time period from starting to print.

Further, set temperatures in correspondence with the respective modes are “190° C.” in the warm up mode or the standby mode, “193° C.” in the print start mode, “180° C.”, “174° C.”, “164° C.” successively in the print2mode through print4mode, “160° C.” in the energy saving mode and “0° C.” in the emergency stop mode.

The set temperature of the energy saving mode is set to a value which is lower than the printable temperature (about 170° C.) but can reach the printable temperature in a short period of time by applying comparatively small energy.

After setting the control target temperature to the variable Tref, an output value (detected temperature) of the first surface temperature sensor107provided via the A/D converter508is stored to a variable Tsns (step704). Next, the value of the variable Tsns (detected temperature) is compared with the value of the variable Tref (control target temperature) (step705). When the detected value is equal to or lower than the control target temperature, the fixing heater105and the temperature holding heater106are lighted by executing a heater lighting sequence (step707). On the other hand, when the detected temperature is higher than the control target temperature, the fixing heater105and the temperature holding heater106are not lighted (step706).

By the above-described temperature control, the fixing roller41aand the pressing roller41bof the fixing unit40are controlled to a pertinent temperature in accordance with the mode.

In the standby mode, the temperature of the fixing roller of the fixing unit40is maintained at 190° C. (first temperature) capable of forming the image and therefore, the image forming can immediately be executed.

Further, in the energy saving mode, the temperature is maintained at 160° C. (second temperature) lower than the first temperature and therefore, power consumption can be restrained to be low. Further, the second temperature is only slightly lower than the printable temperature and therefore, when the image forming start signal is inputted, energy required for recovering from the energy saving mode can be reduced and a power consumption efficiency as a whole can also be promoted. Further, the mode can be recovered in the short period of time and therefore, a recovery waiting time period can also be reduced, which amounts to promote convenience.

Further, since the fixing unit40includes two heating sources of the fixing heater105and the temperature holding heater106, it is preferable to use the two heaters for heating the respective rollers in the standby mode, the print mode or the like having high set temperatures and use only one of the heaters (for example, fixing heater105) in the energy saving mode having a low set temperature. By reducing a number of the heating sources of supplying power, power consumption in the energy saving mode can further be reduced.

FIG. 10shows an example of the interval table TblRotIntvl (605ofFIG. 7) stored in the ROM505. The table defines relations between the modes of the image forming apparatus and the executing intervals of the nip position change operation. According to the example, a value of “5 minutes” is set as an executing interval in the standby mode and a value of “10 minutes” is set as an executing interval in the energy saving mode.

FIG. 11is a flowchart showing a nip position change control algorithm. The processing is executed by the CPU501(program) as an executing unit repeatedly at constant time intervals. The function of the CPU501corresponds to control means as the example.

First, it is confirmed whether the current mode of the image forming apparatus is the standby mode or the energy saving mode (step1001).

When the mode is not the standby mode or the energy saving mode, there is brought about a state in which the fixing motor is rotated in printing or in warming up, or a state in which the fixing heater is not lighted by the emergency stop of jamming or the like. In this case, it is not necessary to execute the nip position change operation (compression set preventing rotation) and therefore, the interval timer TimeRotIntvl is reset to 0 and the operation is finished (step1003).

In the case of the standby mode or the energy saving mode, the interval timer TimeRotIntvl is incremented by 1 (step1002). Further, the processing branches depending on whether the mode is the standby mode or the energy saving mode (step1004).

In the case of the standby mode, an interval for the standby mode is read from the interval table TblRotIntvl and the interval value of “5 minutes” is set to the nip position change interval TimeUpRotIntvl (step1008). Successively, it is determined whether the value of the interval timer TimeRotIntvl reaches the value of the nip position change interval TimeUpRotIntvl (step1009), and when the value is reached, the fixing motor is rotated for 1 second (step1015), the interval timer TimeRotIntvl is reset and the operation is finished (step1014). However, when the value of the interval timer TimeRotIntvl is smaller than the value of the nip position change interval TimeUpRotIntvl, the operation is finished as it is.

By such a processing, in the standby mode, the nip position change operation is executed at the interval of 5 minutes and the compression set is restrained from being brought about.

Although the set value to the nip position change interval TimeUpRotIntvl is described as “5 minutes” to facilitate to understand the explanation, in an actual program, a “value corresponding to 5 minutes” is set in accordance with a unit of counting by the interval timer TimeRotIntvl. For example, when the processing ofFIG. 11is executed once per second, the time is counted up for each second and therefore, the nip position change interval TimeUpRotIntvl is set with a value of “300 (=5×60 seconds)”.

(Case of Energy Saving Mode)

In the case of the energy saving mode, first, it is determined whether the value of the interval switch timer TimeRotChng reaches the value of the switch time period TblRotChng of the nip position change interval (step1005). The interval switch timer TimeRotChng is a value measured by an elapsed time measuring algorithm, mentioned later, and corresponds to an elapsed time period after switching from the standby mode to the energy saving mode. Further, the switch time period TblRotChng is a threshold for defining a timing of changing the nip position change interval TimeUpRotIntvl. By the processing of step1005, it is determined whether the elapsed time period from the time point of switching from the standby mode to the energy saving mode elapses for 30 minutes or longer.

When the elapsed time period is less than 30 minutes, the CPU501(functioning as a changing unit) reads an interval for the standby mode from the interval table TblRotIntvl and sets the interval value of “5 minutes” to the nip position change interval TimeUpRotIntvl (step1006).

When the elapsed time period is 30 minutes or longer, the CPU501sets an interval value of “10 minutes” for the energy saving mode (step1007). That is, the nip position change operation is executed at the interval (5 minutes) equal to that of the case of the standby mode during a time period until 30 minutes is elapsed since the mode has been switched to the energy saving mode and thereafter at the interval (10 minutes) longer than the case of the standby mode thereafter.

Successively, it is determined whether the value of the interval timer TimeRotIntvl reaches the value of the nip position change interval TimeUpRotIntvl (step1010). When the value is reached, first, the drive power source for the fixing motor is started up (step1011), the fixing motor is rotated for 1 second (step1012) and thereafter, the drive power source is shut off again (step1013). Further, the interval timer TimeRotIntvl is reset and the operation is finished (step1014). On the other hand, when the value is not reached, the operation is finished as it is without executing the nip position change operation.

Since a time period of rotating the fixing motor in the nip position change operation is set to 1 second, the fixing roller is rotated by one eighth turn and the nip position is properly changed. That is, the rotational angle of the fixing roller in the nip position change operation is about 45°, which is smaller than 360°.

By such a nip position change control, the nip position of the fixing roller and the pressing roller is shifted at each predetermined time interval and therefore, the compression set is restrained from being brought about.

<Elapsed Time Period Measuring Algorithm>

FIG. 12is a flowchart showing an elapsed time period measuring algorithm. The processing is executed by the CPU501(program) repeatedly at each constant time period. The function of the CPU501corresponds to a measuring unit of the example.

First, it is determined whether the mode of the image forming apparatus is the energy saving mode (step1101). In the case of the energy saving mode, 1 is added to the value of the interval switch timer TimeRotChng (step1102). On the other hand, when the mode is not the energy saving mode, the interval switch timer TimeRotChng is reset 0 (step1103). By the processing, the elapsed time period from switching the mode can be measured.

According to the above-described constitution of the embodiment, after shifting from the standby mode to the energy saving mode, the nip position change operation is executed in accordance with the actual temperature of the fixing roller and therefore, the compression set can effectively be restrained. Further, power consumption can be reduced since the drive power source of driving the fixing motor is shut off in the energy saving mode. Further, after the temperature of the fixing roller is lowered, the frequency of the nip position change operation is reduced and therefore, deterioration in the power source parts by repeating to start up/shut off the drive power source can be minimized.

Further, the above-described embodiment only exemplifies a specific example of the present invention. The scope of the invention is not limited to the above-described embodiment but includes various modifications within the scope of the technical ideas thereof.

For example, although according to the above-described embodiment, only one stage of switching the nip position change interval is carried out in the energy saving mode, it is preferable to switch the interval by two stages or more by increasing a number of data of the interval table TblRotIntvl (for example, 5 minutes, 8 minutes, 10 minutes).

According to the above-described embodiment, the nip position change operation can efficiently and properly be executed in the energy saving mode and therefore, the restraint of the compression set and the reduction of the power consumption can be made to be compatible, further, an effect of reducing the load on the power source parts can be expected.

This application claims priority from Japanese Patent Application No. 2004-167231 filed Jun. 4, 2004, which is hereby incorporated by reference herein.