JP2006243055A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable a direct tandem type image forming apparatus to always perform excellent image formation by prolonging the life of a conveying transfer belt. <P>SOLUTION: The image forming apparatus 10 has a transfer belt 11 which conveys a recording medium in a designated direction, a plurality of image formation units having photoreceptor drums 16a to 16d which are arranged in the moving direction of the transfer belt and carry toner images by colors, and transfer rollers 17a to 17d which face the photoreceptor drums across the transfer belt and transfer toner images on the photoreceptor drums to the recording medium. When the transfer belt is cleaned, a bias voltage having the opposite polarity from a bias voltage applied to transfer the toner images on the photoreceptor drums to the recording medium is applied as a cleaning bias to at least one of the transfer rollers. At this time, a reflection type sensor 18 detects the amount of toner sticking on the transfer belt surface as a dirt state and based upon the output of the detection sensor, it is determined whether or not the transfer belt is cleaned. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus using an electrophotographic process such as a copying machine, a printer, or a facsimile machine, and more particularly to a tandem type color image forming apparatus.

  In general, as a full-color image forming apparatus using an electrophotographic process, a toner image is sequentially superimposed and transferred (primary transfer) onto an intermediate transfer belt using an intermediate transfer belt (intermediate transfer member), and a color toner image is transferred. After formation, a color toner image on an intermediate transfer belt is transferred to a recording medium (recording paper, hereinafter simply referred to as paper) at a secondary transfer position to form a color image (such color The image forming apparatus is called an intermediate transfer tandem type image forming apparatus).

  On the other hand, there is also a type in which color images are formed by sequentially superimposing and transferring toner images onto a sheet conveyed on a sheet transfer conveyance belt (such a color image forming apparatus directly forms a tandem type image formation). Called device). In the following description of this section, the intermediate transfer belt and the sheet transfer conveyance belt will be collectively referred to as a transfer belt.

  By the way, the above-mentioned transfer belt is soiled by toner adhesion or the like, and is therefore cleaned by a transfer belt cleaner. However, the transfer belt is contaminated by paper dust and discharge products after long-term use. If the surface of the transfer belt is contaminated, the surface resistance is lowered, the transfer performance is lowered, and good image formation cannot be performed.

  For this reason, when the surface of the transfer belt is measured by an optical sensor and it is determined that the surface property of the transfer belt is deteriorated, the metal polishing roller is brought into contact with the transfer belt to rotate the transfer belt, and the polishing roller Is rotated in the reverse direction at the same speed as the moving speed of the transfer belt, and discharge products adhering to the surface of the transfer belt and contaminants such as paper dust are peeled off from the belt surface (see Patent Document 1). ).

  Further, before the predetermined pattern image formed on the transfer belt is read by the pattern detection sensor and the resist correction is performed according to the pattern detection signal, the transfer belt is rotated once to detect scratches and dirt on the transfer belt. The pattern detection sensor reads the amount of light reflected from the transfer belt, detects the degree of scratches and dirt on the transfer belt according to the pattern detection signal at that time, and displays an instruction to replace the transfer belt according to the result. There is something like that (see Patent Document 2).

JP 2003-302878 (pages 5 to 6, FIGS. 7 to 8) Japanese Patent Application Laid-Open No. 2003-241472 (pages 5 to 7, FIGS. 3 to 6)

  However, in the image forming apparatus described in Patent Document 1, when the surface state of the transfer belt is detected by an optical sensor and the surface property of the transfer belt is determined to be deteriorated, the image is adhered to the transfer belt by the polishing roller. The purpose of this product is to remove contaminants such as electric discharge products and paper dust. This polishing roller is not intended to remove toner adhering to the transfer belt, and is further transferred by polishing with a polishing roller. The belt may be damaged, and an image defect may occur during image formation.

  Further, in the image forming apparatus described in Patent Document 2, since the transfer belt is only instructed in accordance with the scratch and dirt on the transfer belt, even if the transfer belt is a consumable item, There is a problem that the transfer belt cannot be used over a long period of time, and the life cannot be extended.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus capable of extending the life of a transfer belt and always performing good image formation in view of the problems of the prior art.

  Therefore, in order to solve such a problem, the present invention has a transfer belt that conveys a recording medium in a predetermined direction, and an image carrier that is arranged along the moving direction of the transfer belt and carries a toner image for each color. An image forming apparatus comprising: a plurality of image forming units; and a transfer unit that transfers the toner image on the image carrier to the recording medium so as to face the image carrier with the transfer belt interposed therebetween. A reverse bias applying means for applying to the at least one of the transfer means, as a cleaning bias, a bias voltage having a reverse polarity to the bias voltage when transferring the toner image on the image carrier to the recording medium when cleaning; And a detection sensor for detecting the toner adhesion amount on the surface of the transfer belt as a dirty state. It is characterized in that so as to determine whether to perform.

  In the present invention, whether or not to clean the transfer belt is determined by comparing the coverage of the transfer belt with the toner obtained from the output from the detection sensor and the coverage threshold. Further, the detection sensor is characterized in that it detects using reflected light from the toner on the transfer belt and reflected light from the transfer belt.

  In the present invention, first to Nth thresholds (N is an integer of 2 or more) are provided as the coverage threshold, and the cleaning bias is changed according to the relationship between the coverage and the first to Nth thresholds. Let

  In the present invention, a temperature / humidity sensor for measuring environmental temperature / humidity may be provided, and the cleaning bias may be changed according to the temperature / humidity measured by the temperature / humidity sensor.

  As described above, the image forming apparatus of the present invention detects whether the toner adhesion amount on the surface of the transfer belt is a dirty state by the detection sensor, and determines whether or not to clean the transfer belt based on the output of the detection sensor. Since the transfer belt is not unnecessarily cleaned, not only can the life of the transfer belt be extended, but also there is an effect that good image formation can always be performed.

  In the present invention, the first to Nth threshold values are provided as the coverage threshold value, and the cleaning bias is changed according to the relationship between the coverage rate and the first to Nth threshold values. The cleaning belt can be cleaned by increasing the cleaning bias, and as a result, the transfer belt can be satisfactorily cleaned.

  In the present invention, since the cleaning bias is changed according to the environmental temperature and humidity, the transfer belt can be cleaned according to the state of the toner adhering to the transfer belt, and as a result, the transfer belt can be satisfactorily cleaned. There is.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, but are merely illustrative examples. Not too much.

  FIG. 1 is a diagram schematically illustrating an example of a direct tandem color printer used as an image forming apparatus. In the illustrated printer 10, the color information of original image data sent from an external computer (not shown) is used. To select either full-color image output or monochrome image output. The printer 10 has a paper transfer and conveyance belt (transfer belt (transfer conveyance body): made of, for example, polyvinylidene fluoride (PVDF)) 11 for conveying paper, and the transfer belt 11 includes a driving roller 12, a driven roller 13, and It is stretched around the auxiliary rollers 14a and 14b, and is driven to rotate in the direction indicated by the solid arrow by the drive roller 12.

  Four image forming units 15M, 15C, 15Y, and 15B are sequentially arranged along the transfer belt 11 from the upstream side in the conveyance direction of the transfer belt 11, and in the illustrated example, in order from the upstream side in the rotation direction of the transfer belt 11. A magenta (M) image forming unit 15M, a cyan (C) image forming unit 15C, a yellow (Y) image forming unit 15Y, and a black (BK) image forming unit 15B are arranged along the transfer belt 11. Yes.

  Each of the image forming units 15M, 15C, 15Y, and 15B includes photoconductor drums 16a to 16d that use amorphous silicon (a-Si) as a photoconductor, and although not shown, around the photoconductor drums 16a to 16d Are provided with a charger, an LED print head unit, a developing device, a cleaning device, and the like. Further, transfer rollers 17a to 17d are arranged so as to face the photosensitive drums 16a to 16d with the transfer belt 11 interposed therebetween.

  When image formation is performed in the color mode, the photosensitive drums 16 a to 16 d are in contact with the transfer belt 11, and a sheet is fed onto the transfer belt 11 from a paper feed cassette (not shown). When the paper on the transfer belt 11 passes under the photosensitive drum 16a, the M toner image formed on the photosensitive drum 16a is transferred to the paper by the transfer bias voltage applied to the transfer roller 17a.

  In this way, after the magenta toner image is transferred to the paper by the image forming unit 15M, the respective color toner images of C, Y, and BK are sequentially transferred to the paper by the image forming units 15C, 15Y, and 15B. The toner images are superimposed and a color image is formed on the paper. Thereafter, the sheet is conveyed to a fixing device (not shown), where the color toner image is fixed and discharged to a discharge unit (discharge tray: not shown).

  On the other hand, when an image is formed in the monochrome mode, the transfer belt unit including the transfer belt 11 is rotated about the axis of the drive roller 12 as shown by the broken line arrow in FIG. -16c are separated from the transfer belt 11, and only the photosensitive drum 16d is in contact with the transfer roller 17d with the transfer belt 11 in between. Then, a transfer bias voltage is applied to the transfer roller 17d, and the black toner image is transferred onto the paper as described above, and then discharged through the fixing device.

  By the way, in the printer 10, in order to perform image density correction, a reflective sensor 18 is disposed opposite to the auxiliary roller 14 a located on the downstream side of the drive roller 12. For example, a predetermined number of prints can be printed. When (image formation) is performed, a toner image for measurement (toner patch) is formed on the transfer belt 11 for each color and each density, and the toner density of the toner patch on the transfer belt 11 is measured. Accordingly, image density correction is performed by correcting and controlling image forming conditions such as a developing bias.

  After the image density correction is performed, a reverse transfer bias voltage having a reverse polarity to the transfer bias is applied to at least the transfer roller 17d among the transfer rollers 17a to 17d by a reverse bias applying device (not shown). 11 is reversely transferred to the photosensitive drum 16d, and the transfer belt 11 is cleaned.

  In general, when the transfer belt 11 is cleaned, for example, the photosensitive drums 16 a to 16 c are separated from the transfer belt 11, and the peripheral speed difference between the photosensitive drum 16 d that contacts the transfer belt 11 and the transfer belt 11 is determined. Then, the transfer belt 11 is cleaned.

  On the other hand, as the printing operation (image formation) is performed, the transfer belt 11 is inevitably contaminated with toner or the like, and when the transfer belt 11 is contaminated, image defects such as image fogging occur. For this reason, when a predetermined number of sheets are printed, a cleaning operation (aging operation) is automatically started to clean the transfer belt 11. Also at this time, as described above, the photosensitive drums 16a to 16c are separated from the transfer belt 11, and a peripheral speed difference is given between the photosensitive drum 16d that contacts the transfer belt 11 and the transfer belt 11, The transfer belt 11 is cleaned.

  However, as described above, when the predetermined number of prints are printed, if the aging operation is automatically performed, the aging operation is performed even though the transfer belt 11 is not dirty. As a result, the number of aging operations increases, and each time the photosensitive drum and the transfer belt are rubbed, the life of the photosensitive drum and the transfer belt is shortened (that is, the durability is deteriorated). ), And during that time, image formation cannot be performed, and the user wasted time.

  Here, in order to extend the life of the photosensitive drum and the transfer belt and always form a good image, the aging operation is controlled as described later. Referring to FIG. 2, the reflective sensor 18 has a light emitting element (for example, LED) 19 that projects measurement light onto the surface of the transfer belt 11, and first and second light receiving reflected light reflected from the transfer belt 11. The polarizing filter 22 is disposed between the light emitting element 19 and the transfer belt 11, and the polarizing filter 22 transmits only P-polarized light.

  On the other hand, a polarization separation prism 23 is disposed between the first light receiving element 20 and the transfer belt 11, and this polarization separation prism 23 transmits P-polarized light to the first light receiving element 20, The S-polarized light is reflected and applied to the second light receiving element 21. As shown in the figure, the light emitting element 19 is disposed at a predetermined angle with respect to the surface of the transfer belt 11.

  Assume that a sufficient amount (appropriate amount) of toner has been transferred onto the transfer belt 11. When the measurement light is projected from the light emitting element 19 onto the transfer belt 11, the measurement light including the P-polarized light P <b> 1 and the S-polarized light S <b> 1 is cut by the polarization filter 22, and the polarization filter 22 transmits the measurement light to the transfer belt 11. All of the P-polarized light P1 projected on the lens 11 is reflected by the toner. That is, when a sufficient amount of toner is transferred onto the transfer belt 11, the light P1 does not pass through the toner and reaches the surface of the transfer belt 11, and the light P1 is reflected only by the toner.

  Here, P-polarized light included in the reflected light reflected by the toner is P3, and S-polarized light is S3. A polarization separation prism 23 is disposed on the optical path of the light that is regularly reflected by the light projected (on the regular reflection optical path opposite to the projected light with respect to the normal direction of the transfer belt 11). Separated into polarized light and S-polarized light.

  As described above, the reflected light is separated by the polarization separation prism 23, and P-polarized light P <b> 3 is applied to the first light receiving element 20, and S-polarized light S <b> 3 is applied to the second light receiving element 21. The first and second light receiving elements 20 and 21 photoelectrically convert the received light and output first and second output signals. These first and second output signals are A / D converted. Is applied to a controller (not shown).

  In the controller, the output levels of the first and second light receiving elements 20 and 21 are set so that the levels of the first and second output signals are equal when a sufficient amount of toner is attached to the transfer belt 11. (Gain) has been adjusted. That is, if the transfer belt 11 has an appropriate toner amount, the levels of the first and second output signals are equal (this output level is referred to as an appropriate toner amount output level). And after adjusting the output level of the 1st and 2nd light receiving element, let the output dark voltage be Po and So, respectively.

  Further, when measurement light is projected from the light emitting element 19 to the transfer belt 11 in a state where a toner image is not formed on the transfer belt 11 (not transferred), P-polarized light P1 and S-polarized light S1 The light S1 is cut by the polarizing filter 22 and only the light P1 is projected onto the surface of the transfer belt 11, and the transfer belt 11 is subjected to the surface property (for example, surface roughness). The reflected light reflected by the surface 11 has P-polarized light and S-polarized light.

  Here, the P-polarized light included in the reflected light reflected from the surface of the transfer belt 11 is P2, and the S-polarized light is S2. This reflected light is separated by the polarization separation prism 23 into P-polarized light P2 and S-polarized light S2. Then, the S-polarized light S 2 is received by the second light receiving element 21, and the P-polarized light P 2 is received by the first light receiving element 20.

  The first and second light receiving elements 20 and 21 photoelectrically convert the received light (P2, S2) and output first and second output signals, and these first and second output signals are expressed as A After being / D converted, it is given to the controller. The controller sets the first and second output signals to the first and second background voltages Pg and Sg, respectively, and (Pg−Po) − (Sg−So) as a reference value when no toner is attached. Set as. As described above, the toner levels on the transfer belt 11 are measured after adjusting the output levels of the first and second light receiving elements 20 and 21 and setting the reference value. The output level adjustment and the reference value setting described above are performed, for example, when the printer is shipped (when the transfer belt 11 is in a new state).

  When printing is performed by the printer 10, the surface of the transfer belt 11 is always measured by the reflective sensor 18, and the first and second light receiving elements 20 and 21 respectively output the first and second outputs. A signal is provided to the controller. Since the reflection type sensor 18 is disposed opposite to the auxiliary roller 14a deviated from the sheet conveyance path, the toner density of the toner image used for forming an image on the sheet is not measured. Specifically, the measurement light including the P-polarized light P1 and the S-polarized light S1 is cut by the polarization filter 22, and only the light P1 is projected onto the toner. Now, assuming that the surface of the transfer belt 11 is soiled with toner, a part of the incident light P1 to the toner is reflected by the surface of the toner, and the rest is transmitted through the toner. The light transmitted through the toner is reflected on the surface of the transfer belt 11.

  That is, the light (light P1) projected onto the surface of the transfer belt 11 is reflected as P-polarized light P2 and S-polarized light S2. The lights P2 and S2 are separated by the polarization separation prism 23, the P-polarized light P2 is received by the first light receiving element 20, and the S-polarized light S2 is received by the second light receiving element 21. Similarly, the reflected light reflected by the toner is separated by the polarization separation prism 23, and P-polarized light P 3 is given to the first light receiving element 20, and S-polarized light S 3 is given to the second light receiving element 21. Given.

  As described above, the first and second light receiving elements 20 and 21 photoelectrically convert the received light to output the first and second output signals, and the first and second output signals are the first and second output signals. After being A / D converted as the first and second measurement signals, they are given to the controller. Now, when the first and second measurement signals are represented by S and P, respectively, the controller obtains (P-Po)-(S-So) as a measurement output value, and sets the measurement output value to the reference value described above. Correct based on. That is, the controller obtains ((P-Po)-(S-So)) / ((Pg-Po)-(Sg-So)) as a correction value, and then uses the correction output value represented by Equation 1 as the coverage ratio. Get as.

  Here, referring to FIG. 3, FIG. 3 is a diagram (hereinafter referred to as a “coverage-stain curve”) showing the relationship between the coverage represented by Equation 1 and the actual toner adhesion amount (stain on the transfer belt 11). It can be seen from the coverage that the toner adhesion amount (that is, the contamination of the transfer belt 11) can be obtained. Then, a coverage threshold value is set in the controller in advance, and when the coverage (calculated coverage) calculated as described above exceeds 0.2, the above-described cleaning operation is executed.

  A state where the coverage ratio = 1 means that the toner adhesion amount is the maximum, and a coverage ratio = 0 means that the toner adhesion amount is zero. The coverage ratio = 0.2 represents about 25% of the maximum (MAX) toner adhesion amount. According to the experiments by the inventors, it was confirmed that when the coverage ratio is 0.2 or less, no image defect occurs and the number of aging operations can be minimized. That is, if the aging operation is performed when the coverage exceeds 0.2, it is possible to prevent image defects and improve the durability (the life of the transfer belt 11 and the like can be extended). it can).

  By the way, when performing the cleaning operation, the cleaning efficiency fluctuates in accordance with the change in the usage environment (particularly temperature and humidity) of the printer 10. As described above, when the transfer belt is cleaned, a reverse transfer bias voltage is applied to the transfer roller to reversely transfer the toner on the transfer belt to the photosensitive drum. The transfer belt may not be sufficiently cleaned.

  For this reason, a temperature / humidity sensor is connected to the controller, and the temperature / humidity in the printer 10 is measured and given to the controller as the detected temperature / humidity. Since the temperature / humidity sensor detects the state of the transfer belt 11, it is disposed in the vicinity of the transfer belt 11 away from a fixing device (not shown). In the controller, for example, the temperature / humidity table shown in FIG. 4 is set as a basic table. When the controller receives the detected temperature / humidity, it refers to the basic table (first table) shown in FIG. A cleaning bias corresponding to the detected temperature / humidity (numerals 725, 775, 825, 850, 875, and 900 (unit volts) in FIG. 4 is the cleaning bias) is selected, and the transfer rollers 17a to 17d At least a reverse transfer bias voltage indicated by a cleaning bias is applied to at least the transfer roller 17d by a reverse bias applying device, and the toner (dirt) on the transfer belt 11 is reversely transferred to the photosensitive drum 16d, and the transfer belt 11 is moved over the entire circumference. Clean it.

  At this time, the controller obtains the coverage expressed by Equation 1 in accordance with the first and second output signals from the reflective sensor 18, but even after the transfer belt 11 is cleaned, the coverage> 0. .2, the cleaning is performed again with reference to the second table (see FIG. 5) preset in the controller. The controller then repeats the cleaning operation with reference to the second table until the coverage becomes 0.2 or less.

  In this way, the transfer belt is cleaned by applying a cleaning bias according to temperature and humidity.If the cleaning is insufficient, the cleaning bias is changed and cleaning is performed again. Even if the charge amount of the toner adhering to the toner is unstable, the cleaning can be performed satisfactorily.

  Further, when the coverage is extremely larger than 0.2 (for example, when a paper jam or the like occurs and the toner image is directly transferred to the transfer belt 11), the controller uses the basic table for the cleaning bias. Cleaning is performed with a value larger than the indicated value, and the transfer belt 11 is cleaned early. In other words, the cleaning bias may be changed according to the contamination of the transfer belt (in other words, the coverage threshold is provided with first to Nth thresholds (N is an integer equal to or greater than 2). The cleaning bias may be changed according to the relationship with the first to Nth threshold values).

  The detection sensor detects the toner adhesion amount on the surface of the transfer belt as a dirty state, and determines whether to clean the transfer belt based on the output of the detection sensor. As a result of not being cleaned and extending the life of the transfer belt as well as being able to always perform good image formation, it can be applied to transfer belt cleaning in a direct tandem image forming apparatus.

1 is a diagram schematically showing Embodiment 1 of an image forming apparatus according to the present invention. It is a figure for demonstrating an example of the reflection sensor used with the image forming apparatus by this invention. It is a figure which shows the relationship between a coverage and an actual toner density | concentration. It is a figure which shows the 1st table which prescribes | regulates the relationship between temperature and humidity, and a cleaning bias. It is a figure which shows the 1st table which prescribes | regulates the relationship between temperature and humidity, and a cleaning bias.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Printer 11 Paper transfer conveyance belt 12 Drive roller 13 Driven roller 15M, 15C, 15Y, 15B Image formation unit 16a thru | or 16d Photosensitive drum 17a thru | or 17d Transfer roller 18 Reflection type sensor 19 Light emitting element 20, 21 Light receiving element 22 Polarizing filter 23 Polarization separation prism

Claims (5)

A plurality of image forming units having a transfer belt that conveys a recording medium in a predetermined direction, an image carrier that is arranged along the moving direction of the transfer belt and carries a toner image of each color, and the transfer belt In an image forming apparatus having transfer means for transferring a toner image on the image carrier to the recording medium facing the image carrier,
When cleaning the transfer belt, a reverse bias application is applied to at least one of the transfer means using a bias voltage having a polarity opposite to that of the bias voltage when the toner image on the image carrier is transferred to the recording medium as a cleaning bias. Means,
A detection sensor for detecting the toner adhesion amount on the surface of the transfer belt as a dirty state,
An image forming apparatus, wherein whether or not to clean the transfer belt is determined based on an output of the detection sensor.   2. The apparatus according to claim 1, wherein whether or not the transfer belt is to be cleaned is determined by comparing a coverage of the transfer belt with toner obtained from an output from the detection sensor and a coverage threshold. Image forming apparatus.   2. The image forming apparatus according to claim 1, wherein the detection sensor detects light using reflected light from toner on the transfer belt and reflected light from the transfer belt.   The coverage threshold is provided with first to Nth (N is an integer of 2 or more) thresholds, and the cleaning bias is changed according to the relationship between the coverage and the first to Nth thresholds. The image forming apparatus according to claim 2. It has a temperature / humidity sensor that measures environmental temperature / humidity,
3. The image forming apparatus according to claim 2, wherein the cleaning bias is changed in accordance with the temperature / humidity measured by the temperature / humidity sensor.

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