Image forming apparatus

An image forming apparatus includes an image bearing member configured to bear a toner image on a surface thereof, a transfer unit configured to transfer the toner image on the image bearing member to a sheet, a fixing unit configured to heat and fix the toner image on the sheet transferred by the transfer unit, a reading unit configured to read the sheet that has passed through the fixing unit, from an end surface of the sheet, a control unit configured to control a fixing temperature when the fixing unit heats and fixes the toner image on the sheet, based on an image read by the reading unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus for forming a toner image on a sheet, and more particularly to an image forming apparatus including a fixing device for heating and fixing the toner image transferred onto the sheet.

2. Description of the Related Art

The conventional image forming apparatuses such as a copy machine and a laser printer include a transfer device for transferring a toner image formed on an image bearing member onto a sheet and a fixing device for heating the sheet onto which the toner image is transferred and fixing the toner image on the sheet.

For example as described in Japanese Patent Application Laid-Open Nos. 63-202776 and 4-16888, in order to stably fix the toner image onto the sheet, the conventional fixing device detects a temperature on a surface of a fixing roller that gives heat to the sheet and controls a heat amount of a heater serving as a heating device.

Various types of environments and paper are tested when the fixing device is developed, and a mutual relation between a preparation temperature on a surface of a fixing roller and a melting state of the toner on the sheet is taken into consideration to determine a preparation temperature of the heater.

However, even when a temperature on a surface of the fixing roller is controlled to be constant, the toner is differently fixed on the sheet due to a change of a surface hardness of the fixing roller or a moisture absorption state of the toner. Accordingly, the toner cannot be stably fixed on the sheet only by detecting the temperature on the surface of the fixing roller and controlling the heat amount of the heater.

SUMMARY OF THE INVENTION

The present invention is directed to an image forming apparatus for stably fixing toner on a sheet and preventing an image from being deteriorated due to poorly fixed toner.

According to an aspect of the present invention, an image forming apparatus includes an image bearing member configured to bear a toner image on a surface thereof, a transfer unit configured to transfer the toner image on the image bearing member to a sheet, a fixing unit configured to heat and fix the toner image on the sheet transferred by the transfer unit, a reading unit configured to read the sheet that has passed through the fixing unit, from an end surface of the sheet, a control unit configured to control a fixing temperature when the fixing unit heats and fixes the toner image on the sheet, based on an image read by the reading unit.

DETAILED DESCRIPTION OF THE EMBODIMENTS

An image forming apparatus according to a first exemplary embodiment of the present invention will be described. According to the present exemplary embodiment, a color image forming apparatus of an electrophotographic type will be described as an example of an image forming apparatus1.

FIG. 1is a cross sectional view of the image forming apparatus1.

A sheet S is fed by a feeding roller3in a sheet storing unit2. The sheet S fed out by the feeding roller3passes through a conveyance path4and is fed to a registration roller5. The registration roller5that is standing still stops the sheet S so that skew of the sheet S is corrected. Then, the registration roller5rotates to feed the sheet S to a second transfer unit6.

The second transfer unit6is a nip unit formed of a second-transfer inside roller7and a second-transfer outside roller8that oppose each other, and a predetermined pressure force and an electrostatic load bias are applied to the second transfer unit6so that a toner image adheres to the sheet S.

The image forming unit includes a photosensitive member9, an exposure device10, a development device11, a first transfer device12, and a photosensitive cleaner13.

The photosensitive member9as an image bearing member rotates in a direction of an arrow as illustrated inFIG. 1, and a surface of the photosensitive member9is evenly charged by a charging device in advance. Then, the charged surface is exposed by an exposure device10via a mirror14according to image information.

More specifically, the exposure device10exposes the evenly charged surface of the photosensitive member9with a laser beam that is ON/OFF controlled (modulation controlled) according to a time-series electric digital image signal of the image information. By this exposure, a potential of the exposed portion on the evenly charged surface of the photosensitive member9is attenuated, and an electrostatic latent image is formed on a surface of the photosensitive drum according to the image information.

The electrostatic latent image formed on the photosensitive member9is developed with the toner by the development device11and becomes visible as the toner image on the photosensitive member9. Thereafter, the predetermined pressure force and the electrostatic load bias are applied by the first transfer device12, and the toner image is transferred onto an intermediate transfer belt15. Then, the toner that slightly remains after the transfer on the photosensitive member9is collected by a photosensitive cleaner13and prepared for the next image forming.

The development device11includes therein development units of yellow (Y), magenta (M), cyan (C), and black (Bk), which are changed according to a latent image color formed on the photosensitive member9to form the toner image on the photosensitive member9. After the toner image is formed, the image having Y, M, C, and Bk is transferred on the intermediate transfer belt15, and color toner images are superimposed. The color toner image transferred onto the intermediate transfer belt15is fed to the second transfer unit6.

After the process of feeding the sheet S and the toner image forming process are performed as described above, a full-color toner image is secondly transferred onto the sheet S in the second transfer unit6. After that, the sheet S is fed to a fixing device17by a conveying belt16.

The fixing device17causes the opposing roller to apply the predetermined pressure force to the sheet S, and causes a fixing heater30in a fixing roller33to apply the heat to the sheet S so that the toner image transferred onto the sheet S can be melted and firmly fixed. The sheet S that has passed through the fixing device17is discharged out of the image forming apparatus1by a discharge roller18.

At a downstream side of the fixing device17in a direction of feeding the sheet, a sheet-end-surface reading device19for detecting fixability of the toner on the sheet S is provided. When the fixability of the toner is detected by using the sheet-end-surface reading device19, a test sheet on which a toner image having predetermined patterns is formed is used.

FIG. 2is a diagram illustrating image patterns on a test sheet20.

As illustrated inFIG. 2, the image patterns having a predetermined width and predetermined spaces therebetween are formed on an end portion of the test sheet20. The sheet-end-surface reading device19reads the image on the end portion of the test sheet20to detect the fixability of the toner on the test sheet20. The details about the fixability will be described below.

When the fixability of the toner is detected by using the test sheet20, the sheet S is skew fed by a skew-feed roller21illustrated inFIG. 1. The skew-fed sheet S is stopped by a stop board23so that a position of the sheet S in a width direction orthogonal to a direction of feeding the sheet S is corrected. By correcting the position in the width direction of the sheet S, the sheet-end-surface reading device19can appropriately read the image on the end surface of the sheet.

Since the image patterns are formed only on the end portion of the test sheet20, there will be no problem if the skew-feed roller21contacts the sheet S before the fixing. However, when an image is formed in an usual process, if the skew-feed roller21contacts a toner image before the fixing, the image will be disturbed. Thus, the skew-feed roller21is separated away from the sheet S.

FIGS. 3 to 6are top plan views each illustrating the fixing device17and the circumstance thereof.

InFIG. 3, the test sheet20is fed in a direction of an arrow A as illustrated the figure. The feeding direction of the test sheet20fed by the conveying belt16is changed by the skew-feed roller21that is skewed at a predetermined angle with respect to the feeding direction A. The test sheet20is, fed in a direction of an arrow B as illustrated inFIG. 4.

Further, as illustrated inFIG. 1, a sheet-thickness detecting sensor22is provided to detect a width of the test sheet20fed by the skew-feed roller21. The sheet-thickness detecting sensor22is a distance measuring sensor of an optical distance measuring type including a light emitting diode and a photo diode. The optical distance measuring type is a conventional formula by which a distance is calculated based on a position where the reflected light that is emitted from the light emitting diode and reflected on a detection target enters the photo diode.

More specifically, when the test sheet20is inserted between the skew-feed roller21and the conveying belt16, the conveying belt16approaches the sheet-thickness detecting sensor22according to the thickness of the test sheet20. The sheet-thickness detecting sensor22measures a distance to the conveying belt16to detect the thickness of the test sheet20before the fixing.

As illustrated inFIG. 5, the test sheet20skew fed by the skew-feed roller21is stopped by the stop board23provided at an end portion of the fixing device17just before entering the fixing device17and the skew is corrected. Then, the test sheet20is fed in the direction of the arrow A again. At this point, the skew-feed roller21is separated from the test sheet20by a skew-feed roller separating unit36as described below.

Thereafter, the test sheet20passes through the fixing device17, and the toner image is heated and fixed. The sheet-end-surface reading device19provided at the downstream side of the fixing device17in a direction of feeding the sheet reads the image patterns formed on the test sheet20from the side surface of the sheet.

When a size of the test sheet20is small (for example, B5 longitudinal feed or A4 longitudinal feed), the stop board23and the sheet-end-surface reading device19move in a direction of an arrow C as described inFIG. 6. With this arrangement, a position of the test sheet20in the width direction orthogonal to the sheet feeding direction can be determined according to the size of the test sheet20. Therefore, the end surface of the test sheet20can be read by causing the test sheet20to contact the sheet-end-surface reading device19.

FIG. 7is a diagram illustrating a configuration of the sheet-end-surface reading device19.

The sheet-end-surface reading device19includes a light emitting diode (LED)24, a transparent stop board25through which light emitted from the LED24passes, a complementary metal-oxide semiconductor (CMOS)26that receives light reflected from the end surface of the test sheet20.

FIG. 8is a control block diagram of the image forming apparatus1.

A central processing unit (CPU)32is an integrated circuit that gives instructions to a fixing/fixation control circuit29and an image-forming control circuit34to control the whole apparatus. The CPU32is connected to a random access memory (RAM)38providing a working area and a read only memory (ROM)39storing a control program.

A thermistor40is a sensor that detects a temperature on a surface of the fixing roller33heated by the fixing heater30in the fixing device17and is disposed on a surface of the fixing roller33. The fixing control circuit29controls the fixing heater30according to the detection results obtained by the thermistor40.

A light emitting unit35and a laser receiving unit37are provided within the sheet-thickness detecting sensor22. The image-forming control circuit34controls light emission of the laser emitting unit35and irradiates the sheet S with a laser beam. The light reflected from the sheet S is received by the laser receiving unit37.

The skew-feed roller separating unit36is provided to separate the skew-feed roller21from the conveying belt16and is formed of solenoid, etc.

The CPU32controls light emission of the LED24provided in the sheet-end-surface reading device19. Light emitted from the LED24falls on the end surface of the sheet and reflected therefrom. The CMOS sensor26receives the reflected light from the end surface of the sheet and acquires the image information of the end surface.

The interface control circuit27stores the image information from the CMOS sensor26. A calculation circuit28calculates a feature amount of the image information stored in the interface control circuit27. The calculation of the feature amount will be described in detail below.

FIG. 10is a flowchart illustrating a control process when a sheet end surface is read.

The ROM39stores the control program for executing the flowchart and the CPU32executes the control program. The CPU32automatically executes the control based on the flowchart when the power of the image forming apparatus1is turned on or the image is adjusted.

Firstly, in step S01, the CPU32gives an instruction to the image-forming control circuit34to form the images on the test sheet20. On the end surface of the test sheet20, the images are formed having a predetermined width and spaces therebetween as illustrated inFIG. 2. Next, in step02, the CPU32detects the thickness of the test sheet20before the fixing in accordance with the detection result of the sheet-thickness detecting sensor22.

Next, in step S03, the CPU32causes the LED24provided in the sheet-end-surface reading device19to light up. In step S04, the CPU32causes the CMOS sensor26to read the image information of the end surface of the test sheet20. When the image information has been read, in step S05, the CPU32causes the LED24to go out. Next, in step S06, the CPU32adjusts a filter gain to make clearer the image on the end surface of the test sheet20.

One example of the image information of the end surface of the test sheet20acquired by executing the above-described process is illustrated inFIG. 9.

In step S07, the CPU32stores the acquired image information in the interface control circuit27and causes the calculation circuit28to calculate the thickness “X” and the width “Y” of the toner31as the feature amount of the image. The thickness “X” of the toner31means a length in a height direction of the toner image and the width “Y” of the toner31means a length in a direction orthogonal to the height direction of the toner image.

In step S08, the CPU32determines whether the thickness of the toner is larger than the threshold value Xth that is previously set. When it is determined that the thickness of the toner “X” is larger than the threshold value Xth in step S08, in step S09, the CPU32gives an instruction to the fixing control circuit29to increase a fixing temperature.

More specifically, when the thickness “X” of the toner is larger than the threshold value Xth, it is considered that the toner has insufficiently melted, which causes poor fixability. Thus, the heater30is controlled to increase the fixing temperature.

When it is determined that the thickness “X” is equal to the threshold value Xth or less in step S08, in step S10, the CPU32determines whether the width “Y” of the toner that is previously set is larger than the threshold value Yth. When it is determined that the width “Y” of the toner is larger than the threshold value Yth in step S10, in step S11, the CPU32gives an instruction to the fixing control circuit29to decrease the fixing temperature.

More specifically, when the width “Y” of the toner is larger than the predetermined value, it is considered that the good image can not be obtained because the toner melts too much and spreads out, which causes the crushed toner. Thus, the CPU32controls the fixing heater30to decrease the fixing temperature.

Upon changing the fixing temperature in steps S09and S11, the CPU32returns to step S01to perform the above-described processing again. On the other hand, when it is determined that the width “Y” of the toner is equal to the threshold value Yth or less in step S10, the CPU32ends this flow.

The above description shows the thickness “X” and the width “Y” of the toner as an example of the feature amount, however, the feature amount is not limited to the embodiment as described above. For example, as illustrated inFIG. 11, an infiltration amount of the melted toner image infiltrating into the test sheet20may be calculated as the feature amount. The toner melted by the heat infiltrates into the sheet. Since the infiltrated portion is colored, the colored portion viewed from the side surface of the sheet is determined as the infiltration amount.

In this case, when the infiltration amount of the melted toner image into the test sheet20is small, it is considered that the toner on the sheet has insufficiently melted, which causes the poor fixability. The CPU32may control the fixing heater30so that the fixing control circuit29increases the fixing temperature.

Further, a shrinkage rate of the sheet may be also used as the feature amount. After passing through the fixing device17, the moisture in the sheet evaporates and the sheet shrinks. If the sheet shrinks too much, quality of the output product is deteriorated. Thus, when the sheet shrinks by more than a predetermined value, the CPU32controls the fixing heater30to decrease the fixing temperature to decrease an amount of shrinkage.

The shrinkage rate can be calculated based on thickness information of the test sheet20before the fixing detected by the sheet-thickness detecting sensor22and thickness information after the fixing detected from the image information of the end surface of the test sheet20.

As described above, according to the image forming apparatus of the first exemplary embodiment, the fixability of the toner is kept constant to prevent the image from being deteriorated due to the poor fixing.

Along with popularization of digital cameras in recent years, it is desired that the image forming apparatus such as a copy machine and a printer using an electrophotographic type for outputting an image can output the image which is as highly glossy as silver halide photography. Therefore, a second fixing device is discussed which is connected to the image forming apparatus and gives gloss as much as the silver halide photography on the sheet output from the image forming apparatus.

FIG. 12is a cross sectional view of the image formation apparatus and a second fixing device according to the second exemplary embodiment. Similar symbols are given to the components having functions similar to the first exemplary embodiment, and the descriptions are not repeated.

The sheet S fixed by a first fixing device50that is disposed inside the image forming apparatus1is discharged out of the image forming apparatus1by the discharging roller18and fed into a second fixing device51.

The sheet S fed to the second fixing device51passes through a conveyance pass60. Then, the end surface of the sheet S, which is guided by a skew-feed roller61, contacts a stop board62before entering an inside of the heating and cooling device52, and skew is corrected similar to the first exemplary embodiment. When the sheet S enters the heating and cooling device52, the skew-feed roller61is separated at the same time.

Next, the heating and cooling device52fixes and cools the sheet S. After the sheet S passes through the heating and cooling device52, the sheet-end-surface reading device19having the configuration similar to the first exemplary embodiment reads the image on the end surface of the sheet S. After a post processing device65performs post processing on, the sheet S is discharged to a discharge tray66.

FIG. 13is a perspective view illustrating a heating and cooling device52in the second fixing device51.

A main part of the heating and cooling device52includes a fixing belt53, a heating roller54, a pressing roller55, a supporting roller56supporting the fixing belt53, a separation roller57, a cooling roller58and a cooling fan59.

The sheet S may be a recording medium applicable to the image forming apparatus1. According to the present exemplary embodiment, in order to acquire a highly-glossy image output, a sheet is formed of double layers including a transparent resin layer made of a thermoplastic resin as a main component on an upper surface of a paper layer. Further, a broken line A illustrates the toner image formed on the resin layer included in the sheet S.

The fixing belt53is an endless belt made of a heat resistant material such as polyamide film as a basic material and be provided with a surface layer whose surface is smooth enough so that the sheet can be easily removed. The heating roller54is a cylindrical metal roller made of aluminum or stainless and includes a surface layer coated with an elastic layer as necessary.

Inside the heating roller54, a heating source64such as a halogen heater is disposed. Further, the heating roller54is rotated by a driving source (not illustrated) to rotate and drive the fixing belt53.

The pressure roller55is a hollow cylindrical roller that is made of aluminum and includes an elastic layer on a surface layer of the pressure roller55. The pressure roller55includes a heating source such as a halogen heater therein as necessary. The pressure roller55, which opposes the heating roller54, is pressed by a pressure member (not illustrated) sandwiched across the fixing belt53to form a nip, and is driven to rotate.

The separation roller57has an outer diameter of Φ 40 mm or less and separates the sheet S that is closely attached to the surface of the fixing belt53and is fed by the fixing belt53due to a curvature and rigidity of the sheet S. The supporting roller56is disposed as a tension roller for applying a tensile force to the fixing belt53or a roller for correcting meandering of the fixing belt53.

The toner image (broken line A) fixed by the pressure roller55and the heating roller54is cooled by wind from the cooling fan59when running on the fixing belt53, and firmly fixed onto the transparent resin layer on the sheet S. Further, the wind applied by the cooling fan59passes through a duct67and cools fins68and69provided respectively on end portions of the separation roller57and the cooling roller58to cool a temperature on surfaces of the separation roller57and the cooling roller58. Thus, the fixing belt53in contact with the separation roller57and the cooling roller58as described above is cooled.

FIG. 14is one example of the image information of the end portion of the test sheet20.

The sheet used in the present exemplary embodiment includes a pulp layer S1and a resin layer S2. The toner image63is infiltrated and firmly fixed into the resin layer S2. The feature amount is obtained from a detected image, for example, a protrusion amount “a” of the toner image63from the resin layer and an infiltration length “b” of the toner image63are obtained. The heating source64is controlled to ensure the better fixability depending on whether the protrusion amount “a” or the infiltration length “b” is within each of predetermined ranges.

Especially, according to the fixing type of the present exemplary embodiment, smoothness of the upper surface of the toner image63is essential to increase gloss of the image. Thus, the protrusion amount “a” is small as much as possible. For example, when the protrusion amount “a” is larger than a predetermined value, the toner has insufficiently melted, which causes insufficient infiltration of the toner. Thus, enough gloss is not acquired. Therefore, in this case, the CPU32controls the heating source64in the heating roller54so that the fixing control circuit29increases the fixing temperature.

Further, when the infiltration length “b” is larger than the predetermined value, it is considered that the good image cannot be obtained because the toner melts too much and spreads out, which causes the crushed toner. Thus, the CPU32controls the heating source64in the heating roller54so that the fixing control circuit29decreases the fixing temperature.

As described above, in the image forming apparatus according to the second exemplary embodiment, the fixability of the toner on the sheet is kept constant similar to the first exemplary embodiment to prevent the image from being deteriorated due to the poor fixing.

In addition, according to the present exemplary embodiment, when the image forming apparatus1is turned on or the image is adjusted, the end surface of the test sheet20is read to control the fixing as described above.

Further, in the above descriptions, the color image forming apparatus using the electronic photograph type as the image forming apparatus1is described as an example, however, the present invention may also be applied to a monochrome image forming apparatus.

This application claims priority from Japanese Patent Application No. 2008-118805 filed Apr. 30, 2008, which is hereby incorporated by reference herein in its entirety.