JP2013109070A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the occurrence of image density unevenness more reliably than before.SOLUTION: An image forming apparatus includes resistance detection means for detecting electrical resistances of primary transfer rollers 25Y, M, C, and K, or electrical characteristic detection means for detecting predetermined electrical characteristics each having correlation with these electrical resistances. The image forming apparatus further includes a control unit that is configured to perform processing to determine proper values of output currents from primary transfer power sources 81Y, M, C, and K on the basis of the detection result by the resistance detection means or the electrical characteristic detection means in addition to the image area ratio of photoreceptors 2Y, M, C, and K.

Description

  According to the present invention, an output target value of a transfer current output means for outputting a transfer current to be supplied to a contact member that contacts the latent image carrier directly or via a belt member is used as a toner image on the latent image carrier. The present invention relates to an image forming apparatus that is updated according to the image area ratio.

  As this type of image forming apparatus, the one described in Patent Document 1 is known. The image forming apparatus includes a photosensitive member as a latent image carrier and a transfer roller that forms a transfer nip in contact with the photosensitive member. The toner image on the photosensitive member is transferred to the recording sheet fed into the transfer nip while applying a transfer bias having a polarity opposite to the normal charging polarity of the toner to the transfer roller.

  A power source that outputs a transfer bias to the transfer roller performs constant current control by adjusting the output voltage so that the transfer current supplied to the transfer roller is equal to a predetermined output target value. Since the appropriate value of the transfer current that can successfully transfer the toner image on the photoconductor to the recording sheet changes according to the image area ratio on the photoconductor in the transfer nip, the power supply can be transferred as follows. The output target value is regularly updated. That is, every time the surface of the photoconductor is sent to the transfer nip by a predetermined length, the average image area ratio on the photoconductor in the length range is calculated, and the output target value is updated based on the calculation result. It is. With this configuration, it is possible to suppress the occurrence of image density unevenness due to the difference in image area ratio.

  However, this image forming apparatus may cause uneven image density. Then, when this inventor researched about the cause, the following things became clear. That is, as a transfer roller as a contact member, in general, a core metal coated with a conductive rubber is used in order to improve the adhesion to the photoreceptor. In such a transfer roller, the electric resistance changes as the environmental change and the deterioration progress. As described above, the appropriate value of the transfer current varies depending on the image area ratio on the photoconductor in the transfer nip. However, the appropriate value of the transfer current also varies depending on the electric resistance of the transfer roller. Has been clarified by experiments of the present inventors.

  The present invention has been made in view of the above background, and an object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of uneven image density as compared with the conventional art.

  In order to achieve the above object, the present invention provides a latent image carrier for carrying a latent image, developing means for developing the latent image on the latent image carrier with toner, and obtaining the toner image, and the latent image carrier. At a transfer nip caused by contact between the latent image carrier and the contact member, or a contact member that contacts the body directly or via a belt member, or contact between the latent image carrier and the contact member. A transfer current is output to the contact member in order to transfer the toner image on the latent image carrier onto the surface of the recording sheet sandwiched in the transfer nip or the belt member, and the latent image Each time the surface of the carrier is sent to the transfer nip by a predetermined length, the output target value of the transfer current is updated to the same value as the appropriate value obtained based on the image area ratio in the predetermined length range. A transfer current output means; In the apparatus, a resistance detection unit that detects an electrical resistance of the contact member or an electrical property detection unit that detects a predetermined electrical characteristic correlated with the electrical resistance is provided, and the appropriate value is set to the image area. In addition to the rate, the transfer current output means is configured to perform processing to be obtained based on a detection result by the resistance detection means or the electrical characteristic detection means.

  In the present invention, the output target value of the transfer current is determined based on both the image area ratio on the latent image carrier in the transfer nip and the electrical resistance of the contact member, so that only the image area ratio is considered. The toner image is transferred at the transfer nip in a state where a transfer current having a more appropriate value is supplied as compared with the conventional case where the output target value is determined. Thereby, generation | occurrence | production of image density nonuniformity can be suppressed compared with the past.

1 is a schematic configuration diagram illustrating a printer according to an embodiment. FIG. 2 is a block diagram showing a part of an electric circuit in the printer. The expansion schematic diagram which shows a chevron patch. The schematic diagram for demonstrating 10 line division on a photoreceptor. FIG. 3 is a schematic diagram illustrating an example of a recording sheet and an image formed on the recording sheet. The partial expansion schematic diagram which shows the image different from FIG. 6 is a graph showing a relationship among a C transfer rate, an M reverse transfer rate, and a primary transfer current when a new C primary transfer roller (electric resistance = 1.4E7 [Ω]) is used. 6 is a graph showing a relationship among a C transfer rate, an M reverse transfer rate, and a primary transfer current when a used C primary transfer roller (electric resistance = 5.6E7 [Ω]) is used. Graph showing an appropriate value I ₁ the first current, and the average image area ratio X, the relation between the electric resistance. Graph showing a second current proper value I _2, the average image area ratio X, the relation between the electric resistance. The graph which shows the relationship between a primary transfer voltage, a primary transfer current, and a test image. The graph which shows the relationship between a primary transfer rate, a primary transfer voltage, M toner reverse transfer rate, and a test image. The graph which shows the relationship between a primary transfer rate, a primary transfer current, M toner reverse transfer rate, and a test image. The graph which shows the IV characteristic of the primary transfer roller whose electrical resistance R is 1.4E7 [(ohm)]. The graph which shows the IV characteristic of the primary transfer roller whose electrical resistance R is 5.64E7 [(ohm)]. FIG. 9 is a schematic configuration diagram illustrating a printer according to a third modification. FIG. 10 is a schematic configuration diagram illustrating a printer according to a fourth modification. FIG. 10 is a schematic configuration diagram illustrating a printer according to a fifth modification.

Hereinafter, an embodiment in which the present invention is applied to a color printer (hereinafter simply referred to as a printer) that forms a color image by a tandem type image forming unit as an image forming apparatus will be described.
First, a basic configuration of the printer according to the embodiment will be described. FIG. 1 is a schematic configuration diagram illustrating a printer according to an embodiment. The printer includes an optical writing unit (not shown), a tandem image forming unit 10, a transfer unit 20, a fixing device 40, a retransmission device 50, and the like. The tandem image forming unit 10 includes four image forming units 1Y, 1M, 1C, and 1K for forming respective color toner images of Y (yellow), M (magenta), C (cyan), and K (black). ing.

  The transfer unit 20 includes an endless intermediate transfer belt 21, a driving roller 22, a driven roller 23, a secondary transfer counter roller 24, four primary transfer rollers 25Y, 25M, 25C, 25K, a secondary transfer roller 26, and the like. doing. An endless intermediate transfer belt 21 as an image carrier is wound around a driving roller 22, a driven roller 23, and a secondary transfer counter roller 24 in such a posture that a side view is an inverted triangular shape. . Then, it is endlessly moved in the clockwise direction in the drawing by the rotational drive of the drive roller 22. In addition to the driving roller 22, the driven roller 23, and the secondary transfer counter roller 24, four primary transfer rollers 25 Y, M, C, and K are also disposed inside the loop of the intermediate transfer belt 20. The roles of the primary transfer rollers 25Y, M, C, K and the secondary transfer roller 26 will be described later.

  The tandem image forming unit 10 is disposed above the transfer unit 20 in a posture in which the four image forming units 1Y, 1M, 1C, and 1K are arranged in a horizontal direction along the overlaid surface of the intermediate transfer belt 21. . The image forming units 1Y, 1M, 1C, and 1K are drum-shaped photoreceptors 2Y, 2M, 2C, and 2K that are driven to rotate counterclockwise in the figure, developing units 3Y, 3M, 3C, and 3K, and charging means. 4Y, M, C, K. It also has drum cleaning devices for Y, M, C and K (not shown). The photoreceptors 2Y, 2M, 2C, and 2K are in contact with the overlaid surface of the intermediate transfer belt 21 to form primary transfer nips for Y, M, C, and K, respectively, and are driven by driving means (not shown). It can be driven to rotate counterclockwise. The developing units 3Y, 3M, 3C, and 3K develop the electrostatic latent image formed on the photoreceptors 2Y, 2M, 2C, and 2K with Y, M, C, and K toners. The charging units 4Y, 4M, 4C, and 4K uniformly charge the surfaces of the photoreceptors 2Y, 2M, 2C, and 2K to the same polarity as the charging polarity of the toner.

  Below the primary transfer nips for Y, M, C, and K, in the loop of the intermediate transfer belt 21, the primary transfer rollers 25Y, M, C, and K place the intermediate transfer belt 21 on the photoreceptors 2Y, M, and C. , Pressing toward K. A primary transfer bias is applied to these primary transfer rollers 25Y, 25M, 25C, 25K by primary transfer power supplies 81Y, 81M, 81C, 81K.

  An optical writing unit (not shown) is disposed above the tandem image forming unit 10. In this optical writing unit, the surface of the photoreceptors 2Y, M, C, and K uniformly charged by the charging means 4Y, M, C, and K is subjected to an optical writing process using the scanning light L to be electrostatically charged. It forms a latent image.

Each of the photoreceptors 2Y, 2M, 2C, and 2K has a drum-shaped substrate having a diameter of 60 [mm] whose surface is coated with an organic photosensitive layer, and the capacitance of the substrate is 9.5E-7 [F. / M ² ]. Then, it is driven to rotate counterclockwise in the figure by a driving means (not shown). The surfaces of the photoreceptors 2Y, M, C, and K uniformly charged by the charging means 4Y, M, C, and K are exposed and scanned by a laser beam emitted from an optical writing unit (not shown), and Y, M, It carries electrostatic latent images for C and K.

  The developing devices 3Y, 3M, 3C, and 3K store a developer (not shown) containing Y, M, C, and K toners and a magnetic carrier. The developer contains a magnetic carrier and a polymerized toner made of a polyester material. An opening is formed in the casing of each of the developing devices 3Y, 3M, 3C, and 3K, and a part of the peripheral surface of the cylindrical developing sleeve is exposed from the opening, and the surface of the photoreceptors 2Y, 2M, 2C, and 2K Opposite to. The developing sleeves for Y, M, C, and K carry the developer in the casing by the magnetic force generated by a magnet roller (not shown) that is fixed inside so as not to rotate with itself. Then, along with its own rotational drive, the developer is transported to a development area where the photoconductor 2Y, M, C, K is opposed to the developer.

In the development region, negative polarity Y, M, C between the developing bias applied to the developing sleeve for Y, M, C, K and the electrostatic latent image of the photoreceptors 2Y, M, C, K. , A developing potential for moving the K toner from the sleeve side to the photosensitive member side acts. Further, negative Y, M, C, and K toners are transferred from the photosensitive member side to the sleeve side between the developing sleeve for Y, M, C, and K and the non-image portion of the photosensitive member 2Y, M, C, and K. The moving non-image potential acts. The Y, M, C, and K toners in the developing devices 3Y, 3M, 3C, and 3K are charged with a charge amount of about −25 [μc / g] by stirring or the like. By the action, it is transferred to the electrostatic latent images of the photoreceptors 2Y, M, C, K. As a result, the electrostatic latent images on the photoreceptors 2Y, 2M, 2C, and 2K are developed to become Y, M, C, and K toner images. The toner adhesion amount per unit area of the Y, M, C, and K toner images on the photoreceptors 2Y, M, C, and K is about 0.43 [mg / cm ² ].

  Each of the developing devices 3Y, 3M, 3C, and 3K has a toner concentration sensor (not shown) that measures the toner concentration of the internal developer. The detection result by the toner density sensor is sent to a control unit (not shown) as a voltage signal. The control unit includes a RAM, in which the output target value of the output voltage from the toner density sensor is stored. Then, the value of the output voltage from the toner density sensor is compared with the output target value, and a Y toner supply device (not shown) is driven for a time corresponding to the comparison result. By this driving, an appropriate amount of Y, M, C, K toner is supplied to the developer whose Y, M, C, K toner density has been reduced by consumption of Y, M, C, K toner accompanying development. For this reason, the toner concentration of the developer in the developing devices 3Y, 3M, 3C, and 3K is maintained within a predetermined range.

  An optical writing unit (not shown) disposed above the tandem toner image forming unit 10 has photoreceptors 2Y, M, and W uniformly charged to about −520 [V] by charging means 4Y, M, C, and K. An electrostatic latent image is formed on the C and K surfaces by performing an optical writing process using the scanning light L. Note that the potential Vl of the electrostatic latent image when the solid image is optically written is approximately −40 [V]. The Y, M, C, and K toner images developed on the photoreceptors 2Y, 2M, 2C, and 2K are placed on the intermediate transfer belt 21 in the Y, M, C, and K primary transfer nip described above. The primary transfer is performed by superimposing on the surface. As a result, a four-color superimposed toner image is formed on the front surface of the intermediate transfer belt 21.

  In this printer, a charging member to which a charging bias is applied by the charging bias power sources 80Y, 80M, 80C, 80K is brought into contact with the photoreceptors 2Y, M, C, K as the charging means 4Y, M, C, K. Alternatively, a device is used in which the photosensitive members 2Y, M, C, and K are uniformly charged by generating a discharge between the charging member and the photosensitive members 2Y, 2M, 2C, and 2K in a state where they are close to each other. Instead of such charging means 4Y, M, C, K, a scorotron charger or the like may be employed.

  A paper cassette (not shown) is disposed below the image forming unit. In the paper feed cassette, a plurality of recording sheets are stored in a stack of sheets, and are fed toward the paper feed path by a paper feed roller provided in the paper feed cassette. Note that the process linear velocity, which is the linear velocity of the photoreceptors 2Y, 2M, 2C, 2C, the intermediate transfer belt 21 and the conveying roller, is set to about 350 [mm / sec].

  A registration roller pair 32 is disposed at the end of the paper feed path. The registration roller pair 32 temporarily stops the rotation of both rollers as soon as the recording sheet is sandwiched between the rollers. Then, the recording sheet is sent out toward a secondary transfer nip described later at an appropriate timing.

  The transfer unit 20 disposed below the image forming units 1Y, 1M, 1C, and 1K includes, in addition to the intermediate transfer belt 21, primary transfer rollers 25Y, M, C, K, and the like disposed in a belt loop. A driven roller 23, a secondary transfer counter roller 24, and the like are included. Further, it also has a secondary transfer roller 26 and a belt cleaning device 28 disposed outside the belt loop.

  The four primary transfer rollers 25Y, M, C, and K in the belt loop are arranged so as to press the intermediate transfer belt 21 that moves endlessly against the photoreceptors 2Y, M, C, and K. Thus, primary transfer nips for Y, M, C, and K where the intermediate transfer belt 21 and the photosensitive members 2Y, 2M, 2C, and 2K come into contact are formed.

  The primary transfer rollers 25Y, 25M, 25C, and 25K are arranged so that their rotational axes are located directly below the rotational axes of the photoreceptors 2Y, 2M, 2C, and 2K. And a conductive sponge roller portion coated on the peripheral surface. The outer shape is 16 [mm], and the diameter of the mandrel is 10 [mm]. The electrical resistance of the conductive sponge roller portion is approximately 1.4E7 [Ω] in the initial state. This electric resistance is a voltage of 1000 [V] applied to the primary transfer roller core in a state where a grounded metal roller having an outer diameter of 30 [mm] is pressed against the conductive sponge roller portion with a force of 10 [N]. Is a value calculated from Ohm's law (R = V / I) from the current I flowing when. In addition, after forming an image on several hundred thousand recording sheets, the electrical resistance of the conductive sponge roller portion of the primary transfer roller was measured and found to be about 5.6E7 [Ω]. Thus, the conductive sponge roller part gradually increases the electrical resistance with use. In addition, the electrical resistance is changed due to environmental changes.

  The primary transfer rollers 25Y, M, C, and K are in contact with resistance measurement rollers 27Y, M, C, and K for measuring the electrical resistance of the rollers, and the primary transfer rollers 25Y, M, C, and K are in contact with the primary transfer rollers 25Y, M, C, and K. Is driven to rotate. The resistance measurement rollers 27Y, M, C, and K made of stainless steel (SUS304) having a diameter of 8 [mm] are connected to the ground via relay switches 28Y, M, C, and K. During a normal image forming operation, the coils of the relay switches 28Y, M, C, and K are not excited and the electrical contacts are cut off, so that the primary transfer rollers 25Y, M, C, and K are electrically connected. Float state. When the coils of the relay switches 28Y, 28M, 28C, 28K are excited and the electrical contacts are connected, the primary transfer current supplied to the primary transfer rollers 25Y, 25M, 25C, 25K is supplied to the relay switches 28Y, 28M, 28C, 28C, It flows to ground through K.

  The primary transfer power sources 81Y, 81M, 81C, 81K are used to primarily transfer toner images on the photoreceptors 2Y, M, C, and K onto the intermediate transfer belt 21 at the primary transfer nips for Y, M, C, and K. In doing so, constant current control is performed to output the primary transfer current by the same value as the predetermined output target value. As a result, a primary transfer current having the same value as the output target value is supplied to the primary transfer rollers 25Y, 25M, 25C, 25K, and Y, M, C, K toner on the photoreceptors 2Y, M, C, K. The image is primarily transferred onto the intermediate transfer belt 21. Further, the primary transfer power supplies 81Y, 81M, 81C, 81C, and 81K have a voltmeter (not shown) that measures the value of the voltage output from itself. When the resistance measurement process described later is performed, the output voltage value is measured while outputting a primary transfer current of 5 [μA]. Then, based on the measurement result and Ohm's law, the electrical resistances of the primary transfer rollers 25Y, M, C, and K are calculated.

  The secondary transfer counter roller 24 disposed on the inner side of the belt loop is disposed so as to sandwich the intermediate transfer belt 21 with the secondary transfer roller 26 disposed on the outer side of the belt loop. Thus, a secondary transfer nip is formed in which the front surface of the intermediate transfer belt 21 and the secondary transfer roller 26 outside the loop come into contact with each other. The registration roller pair 32 described above sends the recording sheet sandwiched between the rollers toward the secondary transfer nip at a timing at which the recording sheet can be synchronized with the four-color toner image on the intermediate transfer belt 21.

  A secondary transfer bias having a polarity opposite to that of the toner is applied to the secondary transfer roller 26. The four-color toner image on the intermediate transfer belt 21 is secondarily transferred collectively to the recording sheet in the secondary transfer nip by the action of the secondary transfer bias and nip pressure. Then, combined with the white color of the recording paper P, a full color toner image is obtained.

  Untransferred toner that has not been transferred to the recording sheet adheres to the intermediate transfer belt 21 after passing through the secondary transfer nip. This is cleaned by a belt cleaning device (not shown).

  A fixing unit 40 is disposed on the left side of the secondary transfer nip in the drawing. The fixing unit 40 forms a fixing nip by contact between a fixing roller 41 containing a heat source such as a halogen lamp and a pressure roller 42 pressed toward the fixing roller 41. The recording sheet that has passed through the secondary transfer nip is separated from the intermediate transfer belt 21 and then fed into the fixing unit 40. Then, in the process of being conveyed from the right side to the left side in the figure while being sandwiched by the fixing nip in the fixing unit 40, the full color toner image is fixed by being heated or pressed by the fixing roller 41. The recording sheet on which the toner image is fixed in this manner is discharged out of the apparatus via a pair of discharge rollers (not shown).

  The recording sheet discharged from the fixing device 40 may be sent to the paper discharge roller pair as it is, or may be sent to the retransmission device 50 without being sent to the paper discharge roller pair. Specifically, when a single-side mode print job for forming an image only on the first surface of the recording sheet is performed, the recording sheet discharged from the fixing device 40 is sent to the pair of discharge rollers without exception. In contrast, when a double-sided mode print job for forming images on both sides of a recording sheet is performed, if the recording sheet discharged from the fixing device 40 carries a toner image only on the first side. Then, it is sent to the retransmission device 50 without being sent to the paper discharge roller pair. However, even in the duplex mode, if the recording sheet ejected from the fixing device 40 carries a toner image on both sides, it is sent to a pair of ejection rollers. Switching between sending the recording sheet after passing through the fixing device 40 to the pair of paper discharge rollers or sending it to the retransmission device 50 is performed by switching the sheet conveyance destination by a switching claw (not shown).

  The retransmitting device 50 switches the recording sheet sent from the fixing device 40 in a switchback manner through the switchback path 51 so that the recording sheet is turned upside down. Thereafter, the recording sheet is sent to the switchback path 52. The recording sheet that has passed through the switchback path 52 is fed into a sheet feeding path for conveying the recording sheet from a sheet feeding cassette (not shown) to the secondary transfer nip. As a result, the image is retransmitted to the secondary transfer nip while being turned upside down.

  In the second half area of the paper feed path, the recording sheet sequentially passes through a resistance measuring roller pair 31 and a registration roller pair 32 described later. The retransmitting device 50 sends the recording sheet to a position upstream of the resistance measuring roller pair 31 in the paper feed path. Therefore, regardless of whether the recording sheet is immediately after being sent out from the paper feed cassette or retransmitted by the retransmission value 50, the resistance measurement roller pair 31 and the registration roller pair in the paper feed path. 32 will always be routed through.

  The registration roller pair 32 corrects the skew of the recording sheet by abutting the leading end of the recording sheet in a state where the rotation of the two rollers is stopped. Thereafter, the two rollers are rotated to hold the leading edge of the recording sheet into the registration nip, but immediately after that, the rotation of the rollers is stopped. Then, the rotation of the roller is resumed at a timing at which the recording sheet can be synchronized with the toner image on the belt at the secondary transfer nip.

  FIG. 2 is a block diagram showing a part of an electric circuit in the printer. In the figure, a control unit 200 as control means includes a CPU 200a (Central Processing Unit) as arithmetic means, a RAM 200c (Random Access Memory) as nonvolatile memory, and a ROM 200b (Read Only Memory) as temporary storage means. The control unit 200 controls the entire apparatus, and various devices and sensors are connected. In the figure, only a part of these devices is shown. The control unit 200 controls driving of each device based on a control program stored in the RAM 200c or the ROM 200b. Further, based on image data (write signal at the time of exposure) sent from an external personal computer or the like, primary transfer current values of Y, M, C, and K are determined, and the determined primary transfer current value and In this way, the Y, M, C, and K primary transfer power supplies 81Y, M, C, and K are controlled. The control unit 200 functions as a transfer current output unit together with the primary transfer power supplies 81Y, 81M, 81C, 81K. The output target value of the primary transfer current output from the primary transfer power supplies 81Y, 81M, 81C, 81K is output as a PWM signal from the control unit 200, and is output to the primary transfer power supplies 81Y, 81M, 81C, 81K. Entered.

  Further, the control unit 200 performs a positional deviation amount correction process immediately after a main power switch (not shown) is turned on or whenever a predetermined number of prints are performed. In this misregistration amount correction process, misregistration detection images made up of a plurality of toner images called chevron patches PV as shown in FIG. 3 are formed on the intermediate transfer belt 21. The optical sensor unit 86 shown in FIG. 2 passes the light emitted from the light emitting means through the condenser lens, reflects the light on the surface of the intermediate transfer belt 21, and receives the reflected light by its own light receiving means. Outputs voltage according to the amount. When the toner image in the chevron patch PV formed on the intermediate transfer belt 21 passes directly below the optical sensor unit 86, the amount of light received by the light receiving means of the optical sensor unit 86 changes greatly. Thereby, the control unit 200 can detect each toner image in the chevron patch PV formed on the intermediate transfer belt 21. As described above, the optical sensor unit 86 functions as an image detection unit in combination with the control unit 200. As the light emitting means, an LED or the like having an amount of light that can generate reflected light necessary for detecting a toner image is used. As the light receiving means, a CCD having a large number of light receiving elements arranged in a straight line is used.

  The controller 200 detects each toner image in the chevron patch PV formed on the intermediate transfer belt 21, thereby detecting the position in the sub-scanning direction (belt moving direction) in each toner image. As shown in FIG. 3, the chevron patch PV is a posture in which the toner images of each color of Y, M, C, and K are inclined by about 45 [°] from the main scanning direction (the direction in which the laser beam scans on the surface of the photoreceptor). The line pattern group is arranged at a predetermined pitch in the belt moving direction which is the sub-scanning direction. For such Y, C, M toner images in the chevron patch PV, the difference in detection time from the K toner image is read. In the figure, the vertical direction of the drawing corresponds to the main scanning direction, and after the Y, M, C, and K toner images are arranged in order from the left, the postures are different from those by 90 [°]. M and Y toner images are further arranged. Based on the difference between the actual measurement value and the theoretical value for the detection time differences tky, tkc, and tkm with respect to K as the reference color, the amount of deviation in the sub-scanning direction of each color toner image, that is, the amount of positional deviation is obtained. Then, based on the misregistration amount, the optical writing start timing for the photoconductor of the optical writing unit (not shown) is corrected, and the misregistration of each color toner image due to the speed fluctuation of the photoconductor and the intermediate transfer belt 21 is corrected. Reduce.

  In FIG. 1, the toner image is not transferred at the Y primary transfer nip located on the most upstream side in the belt movement direction among the primary transfer nips for Y, M, C, and K. The Y toner image on the photoreceptor 2Y is transferred to the intermediate transfer belt 21 in the state. That is, in the primary transfer nip for Y, the first transfer process without overlay transfer is performed. On the other hand, in the primary transfer nip for M, C, and K, the second transfer process for transferring the toner image on the photosensitive member superimposed on the toner image already transferred onto the intermediate transfer belt 21. Is implemented.

  Primary transfer power supplies 81Y, 81M, 81C, 81K for applying a transfer bias to the intermediate transfer belt 21 via primary transfer rollers 25Y, 25M, 25C, 25K for Y, M, C, and K are respectively provided. A transfer current having the same value as a predetermined output target value is output. The primary transfer power supplies 81Y, 81M, 81C, and 81K each determine an output target value based on the image area ratio in the main scanning direction (photosensitive member axial direction) on the photosensitive member at and near the transfer nip outlet. The image area ratio is sent from the control unit 200 to the primary transfer power sources 81Y, 81M, 81C, 81K. Hereinafter, a method of updating the transfer current output target value based on the image area ratio at the transfer nip exit is referred to as a DTCC (Dynamic Transfer Current Control) method.

  As shown in FIG. 4, the surface of the photosensitive member is theoretically determined by the control unit 200 in the sub-scanning direction (photosensitive member surface moving direction) for each region of 10 pixels with reference to the top of the page. A division is made. Each section by division (hereinafter referred to as “10 line section”) includes 10 pixel lines each consisting of a set of pixels arranged in a straight line in the main scanning direction. For each pixel line, the ratio of the number of pixels in the latent image portion to the total number of pixels is obtained as the image area ratio. Then, the average value of the image area ratios of the ten pixel lines is obtained as the average image area ratio in the “10 line section”. The control unit 200 outputs information on the average image area ratio thus obtained for each of Y, M, C, and K to the primary transfer power supplies 81Y, 81M, 81C, and 81K.

  The primary transfer power supplies 81Y, 81M, 81C, 81K, and 81C output average values of the primary transfer current, and an average image area ratio of “10 line sections” passing through the transfer nip outlet among a plurality of “10 line sections”. It will be decided according to Then, while the “10 line section” is passing through the primary transfer nip outlet, the output voltage is adjusted so as to output a transfer current having the same value as the output target value. When the pixel line on the most downstream side in the “10 line section” passes through the transfer nip outlet, the output target value of the transfer current from the primary transfer power supply corresponds to the average image area ratio of the next “10 line section”. Change to something. The timing of this change is determined based on a timing signal sent from the control unit 200.

  The reason why the output target value of the primary transfer current is determined based on the average image area ratio in the vicinity of the primary transfer nip exit is as follows. That is, the photosensitive member is in a state where the potential of the background portion is higher than that of the electrostatic latent image. At the primary transfer nip exit, peeling discharge occurs between the background portion of the photoreceptor and the intermediate transfer belt 21. When most of the primary transfer current is due to the peeling discharge, the potential of the intermediate transfer belt at the exit of the primary transfer nip does not become so high, so the potential difference between the electrostatic latent image and the intermediate transfer belt is Insufficient transfer results in poor transfer. Therefore, it is assumed that, for example, the average image area ratio on the photoreceptor at the exit of the primary transfer nip is about 50 [%] so that the output voltage can increase the potential of the intermediate transfer belt 21 to an appropriate value. Assume that the output target value of the primary transfer current is set. Then, when the average image area ratio at the nip exit is lower than 50 [%] and the area of the photoreceptor background portion at the nip exit becomes larger, the amount of current consumed by the peeling discharge increases and the intermediate transfer belt 21 Lowering the potential causes transfer defects. Further, when the average image area ratio becomes higher than 50% and the area of the photoreceptor background portion at the nip exit becomes smaller, the amount of current consumed by the peeling discharge decreases and the potential of the intermediate transfer belt 21 increases. Therefore, a discharge is generated between the belt and the electrostatic latent image. This discharge reversely charges the toner, causing a transfer failure. By passing a transfer current according to the average image area ratio in the vicinity of the primary transfer nip exit, an appropriate amount of current is passed through the electrostatic latent image on the photosensitive member, and the potential difference between the electrostatic latent image on the photosensitive member and the belt is reduced. It becomes possible to make it smaller than the discharge start voltage.

  FIG. 5 is a schematic diagram illustrating an example of a recording sheet and an image formed on the recording sheet. The illustrated recording sheet is A3 size plain paper, and is conveyed in the direction of arrow A in the drawing within the printer. In the primary transfer nip, the arrow A direction is the same as the sub-scanning direction. A strip-like image extending in the sub-scanning direction is formed on the recording paper, and the length of this image in the main scanning direction (left-right direction in the figure) is 29.7 [mm]. The width of the A3 size recording sheet is 297 [mm]. Since the image extends over the entire area of the recording sheet in the sub-scanning direction, the image area ratio remains constant at 10 [%] regardless of the position in the sub-scanning direction. That is, when outputting the illustrated image, the average image area ratio in the “10 line section” entering the primary transfer nip exit is 10 [%]. Therefore, when outputting this image, unlike the current waveform of FIG. 4, a constant primary transfer current continues to be output from the leading edge of the image to the trailing edge.

  FIG. 6 is a partially enlarged schematic view showing an image different from FIG. The length of this image is not constant in the main scanning direction, and the length varies depending on the position in the sub-scanning direction. In the illustrated image area, the image area ratio is 100 [%] for the five pixel lines from the top in the sub-scanning direction among the ten pixel lines. On the other hand, the image area ratio is 50 [%] in each of the five pixel lines on the rear end side. In such a “10 line section”, the average image area ratio is 75 [%], so the output target value of the primary transfer current is determined to a value corresponding to 75 [%]. In the embodiment, the average image area ratio is calculated based on a laser writing signal in the optical writing unit.

Next, an experiment conducted by the inventor will be described.
The inventor prepared a printer testing machine having the same configuration as the printer according to the embodiment shown in FIG. As the C primary transfer roller 25C, a new one and a used one were prepared. When the electric resistance of the new primary transfer roller 25C was measured, it was 1.4E7 [Ω]. Further, when the electrical resistance of the used primary transfer roller 25C was measured, it was 5.6E7 [Ω]. The experiment was performed in a state where a new primary transfer roller 25C was set and in a state where a used primary transfer roller 25C was set instead of a new one.

  Two test images were printed by a printer tester. The first test image has a magenta image portion having a dimension in the sub-scanning direction of 6 [mm] and a dimension in the main scanning direction of 14.85 [mm], a dimension in the sub-scanning direction of 6 [mm], and the main scanning direction. And a cyan image portion having a dimension in the scanning direction of 282.15 [mm]. The first test image is a pattern in which a plurality of two-color side-by-side portions in which the magenta image portion and the cyan image portion are arranged in the main scanning direction are arranged on the condition that a space of 6 [mm] is provided in the sub-scanning direction. In the first test image, the image area ratio of the magenta image portion in the two-color side-by-side portion is 5 [%]. The image area ratio of the cyan image portion in the two-color side-by-side portion is 95 [%]. These two image portions are arranged in the main scanning direction so as not to overlap each other. Hereinafter, the two-color side-by-side portion is also referred to as an M5% C95% image portion.

  The second test image has a magenta image portion having a dimension in the sub-scanning direction of 6 [mm] and a dimension in the main scanning direction of 14.85 [mm], a dimension in the sub-scanning direction of 6 [mm], and the main scanning direction. And a cyan image portion having a dimension in the scanning direction of 14.85 [mm]. The second test image is a pattern in which a plurality of two-color side-by-side portions in which the magenta image portion and the cyan image portion are arranged in the main scanning direction are arranged under a condition that a space of 6 [mm] is provided in the sub-scanning direction. In the second test image, the image area ratio of the magenta image portion in the two-color side-by-side portion and the image area ratio of the cyan image portion are both 5%. These two image portions are arranged in the main scanning direction so as not to overlap each other. Hereinafter, the two-color side-by-side portion is also referred to as an M5% C5% image portion.

  When the first test image and the second test image are individually printed, the Y and K primary transfer power supplies 81Y and 81K do not output the primary transfer current, and the Y and K photoconductors 2Y and 2Y K was separated from the intermediate transfer belt 21.

  While measuring the output current value from the C primary transfer power supply 81C, the output current value was gradually increased, and a test image was printed under each output current value condition. Further, in each print, the toner adhesion amount per unit area of the cyan image portion before entering the primary transfer nip on the C photoconductor 2C and the photoconductor remaining on the photoconductor after passing through the primary transfer nip. The toner adhesion amount per unit area in the untransferred cyan image portion was measured. The ratio of the former toner adhesion amount minus the latter toner adhesion amount to the former was determined as the primary transfer rate.

  Further, the toner adhesion amount per unit area of the magenta image portion on the intermediate transfer belt 21 before entering the C primary transfer nip, and the non-image portion after passing through the primary transfer nip on the C photoconductor 2C. The adhesion amount per unit area of the M toner reversely transferred to was measured. The ratio of the latter to the former was determined as the M toner reverse transfer rate. The amount of toner remaining on the photoconductor after passing through the primary transfer nip or the reversely transferred toner was measured based on the spectroscopic measurement result by a reflection spectrodensitometer X-Rite 938. The toner adhesion amount on the intermediate transfer belt 21 and the toner adhesion amount on the photosensitive member before entering the primary transfer nip are sucked from the belt or the photosensitive member by a trek type small charge amount measuring device Model 210-HS. It was calculated from the mass per unit area of the toner.

  FIG. 7 is a graph showing the relationship among the C transfer rate, the M reverse transfer rate, and the primary transfer current when a new primary transfer roller 25C (electric resistance = 1.4E7 [Ω]) is used. FIG. 8 is a graph showing the relationship between the C transfer rate, the M reverse transfer rate, and the primary transfer current when the used primary transfer roller 25C (electric resistance = 5.6E7 [Ω]) is used. . It has been confirmed by other experiments and simulations that the difference between the relationship shown in FIG. 7 and the relationship shown in FIG. 8 is caused only by the difference in electrical resistance of the primary transfer roller 25C.

  As shown in FIG. 7, when a new primary transfer roller 25C (electric resistance = 1.4E7 [Ω]) is used, in the process of gradually increasing the primary transfer current, about 15 [ It can be seen that the M reverse transcription rate starts to increase suddenly when it is increased to μA]. At this time, the C transfer rate of the M5% C95 [%] image portion is about 93 [%]. Thereafter, when the primary transfer current is further increased, the M reverse transfer rate of the M5% C95 [%] image increases to about 5 [%] when it is increased to about 22 [μA]. Further, the C transfer rate of the M5% C5 [%] image portion under this current condition is about 91 [%]. The primary transfer current value that maximizes the C transfer rate of the M5% C5% image area is 58 [μA]. The primary transfer current value that maximizes the C transfer rate of the M5% C95% image area is 34 [μA].

  On the other hand, as shown in FIG. 8, when the used primary transfer roller 25C (electric resistance = 5.6E7 [Ω]) is used, in the process of gradually increasing the primary transfer current, It can be seen that the M reverse transcription rate starts to increase suddenly when it is increased to the vicinity of [μA]. The C transfer rate at this time is about 95 [%]. Thereafter, when the primary transfer current is further increased, the M reverse transfer rate of the M5% C95 [%] image increases to about 5 [%] when it is increased to about 28 [μA]. Further, the C transfer rate of the M5% C5% image portion under this current condition is about 94 [%]. The primary transfer current value that maximizes the C transfer rate of the M5% C5% image area is 42 [μA]. The primary transfer current value that maximizes the C transfer rate of the M5% C95% image area is 30 [μA].

  As described above, the primary transfer current value in the primary transfer nip for C that maximizes the C transfer rate differs depending on the electric resistance of the primary transfer roller 25M in addition to the average image area ratio at the primary transfer nip exit. It is. For the primary transfer nip for M, C, and K, the primary transfer current takes into account not only the transfer rate of the M, C, and K image portions but also the reverse transfer rate of the Y, M, and C image portions. However, the primary transfer current value at which the reverse transfer rate can be kept constant is also determined by the electrical resistance of the primary transfer roller in addition to the average image area ratio at the primary transfer nip exit. Will also be different. That is, the appropriate value of the primary transfer current varies depending not only on the average image area ratio on the photosensitive member at the primary transfer nip exit but also on the electrical resistance of the primary transfer roller. In this specification, only the experimental results of measuring the C transfer rate and the M reverse transfer rate in the C primary transfer nip are shown, but the M transfer rate and the Y reverse transfer in the M primary transfer nip are shown. The rate was similar. The K transfer rate and C reverse transfer rate in the primary transfer nip for K were also similar.

  The above experiment was conducted in a laboratory environment controlled at a temperature of 23 [° C.] and a humidity of 50 [%]. In this case, the fluctuation of the electrical resistance of the primary transfer roller occurs only due to deterioration due to use over time. However, the indoor environment greatly fluctuates under the user. When the temperature and humidity change, the electric resistance of the primary transfer roller also changes accordingly. For this reason, even when a primary transfer roller that is not so deteriorated is used under the user's circumstances, the electrical resistance of the primary transfer roller varies depending on the environment, and accordingly, the appropriate value of the primary transfer current also varies. fluctuate.

  Next, the present inventor has examined an appropriate value of the primary transfer current supplied to the Y primary transfer roller 25Y disposed at the most upstream. To the Y primary transfer nip, the toner image of the other color that has been primarily transferred onto the intermediate transfer belt 21 at the upstream side does not enter, so the Y photoconductor 2Y. In contrast, toner images of other colors are not reversely transferred. Therefore, it is possible to determine an appropriate value of the primary transfer current by taking into consideration only the Y transfer rate without considering the reverse transfer rate.

  In the primary transfer nip for Y, almost the same result as that of the primary transfer nip for C can be obtained. Therefore, the electrical resistance of the primary transfer roller is 1.4E7 [Ω], and When the image area ratio of the Y toner image is 5 [%], as shown in FIG. 7, the output target value may be 58 [μA] at which the maximum transfer rate is obtained. Further, when the electric resistance of the primary transfer roller is 1.4E7 [Ω] and the image area ratio of the Y toner image is 95 [%], as shown in FIG. The output target value may be 34 [μA] at which the transfer rate is obtained. Further, when the electric resistance of the primary transfer roller is 5.6E7 [Ω] and the image area ratio of the Y toner image is 5 [%], as shown in FIG. The output target value may be 42 [μA] at which the transfer rate is obtained. Further, when the electrical resistance of the primary transfer roller is 5.6E7 [Ω] and the image area ratio of the Y toner image is 95 [%], as shown in FIG. The output target value may be 30 [μA] at which the transfer rate is obtained.

9, the first current proper value I ₁ is a proper value of the primary transfer current for Y, which is a graph showing the average image area ratio X, the relation between the electric resistance. Based on the change in the slope of the graph due to the difference in electrical resistance R, the formula for obtaining the _first appropriate current value I ₁ was derived as follows.
_{"I 1 = (3.17E-13R-} 3.11E-5) X + (- 3.97E-13R + 6.49E-5) "
Hereinafter, this equation is referred to as the first equation.

Next, the inventor examined the appropriate value of the primary transfer current supplied to the primary transfer rollers 25M, C, and K for M, C, and K disposed downstream of Y. In addition to the M, C, and K toner images, the Y, M, and C toner images obtained in the upstream primary transfer process enter the M, C, and K primary transfer nips. Therefore, M, C, for the primary transfer second current proper value I ₂ is the appropriate value of the current in the primary transfer nip for K, M, C, other K transfer rate, Y, M, C reversed It is necessary to determine the copy rate in consideration.

10, M, C, and the second current proper value I ₂ is the appropriate value of the primary transfer current for KY, is a graph showing the average image area ratio X, the relation between the electric resistance. From the graphs shown in FIG. 7 and FIG. 8, when the image area ratio is in the range of 5 to 95 [%] and the electrical resistance of the primary transfer roller is 1.4E7 [Ω], a high transfer ratio is obtained. The primary transfer current value that can achieve both a low reverse transfer rate and about 15 to 22 [μA]. Further, when the image area ratio is in the range of 5 to 95 [%] and the electrical resistance of the primary transfer roller is 5.6E7, a primary that can achieve both a high transfer ratio and a low reverse transfer ratio. The transfer current value is about 15 to 28 [μA]. Based on this, the formula for obtaining the _second appropriate current value I2 was derived, and the following was obtained.

_{"I 2 = (- 2.86E-12R} -1.20E-4) X + 1.4E-5 "
However, this is an expression applied when “0 ≦ X <0.05”. Hereinafter, this equation is referred to as a second equation.

Under the condition of “0.05 ≦ X ≦ 0.95”, the formula for obtaining the second appropriate current value I ₂ is as follows.
“I ₂ = (− 7.94E−13R−6.67E−6) X + (1.47E−13R + 2.03E−5)”
Hereinafter, this equation is referred to as a third equation.

Under the condition “0.95 <X ≦ 1,” the formula for obtaining the second current appropriate value I ₂ is as follows.
“I ₂ = (− 1.43E−12R + 1.20E−4) X + (1.43E−12R−1.00e−4)”
Hereinafter, this equation is referred to as a fourth equation.

As can be seen from the first equation, for the most upstream Y, the Y transfer rate is increased by decreasing the first current appropriate value I ₁ as the electrical resistance of the primary transfer roller increases. Note that if the primary transfer current value is reduced even though the electrical resistance of the primary transfer roller is low, an effective transfer current does not flow to the image portion at the nip exit, thus causing a transfer failure. Therefore, it is necessary to decrease the first appropriate current value I ₁ as the electrical resistance of the primary transfer roller increases.

FIG. 7 shows the case where the primary transfer roller used in the experiment is used in a configuration in which the primary transfer current is constantly controlled at a constant output target value as in a conventional general image forming apparatus. From the result, the output target value is generally set to about 44 [μA]. Even in this case, even if the electrical resistance and the image area ratio of the primary transfer roller fluctuate, a transfer rate of about 95 [%] can be obtained. However, if the output target value is updated to the same value as the first appropriate current value I ₁ obtained by the first equation, a transfer rate of 97 [%] or more can always be maintained. Further, in a configuration in which the output target value is updated based on only the average image area ratio X without taking into account the electrical resistance of the primary transfer roller, the transfer ratio is 1% or more than when the first formula is employed. Lower. In addition to the fact that the image density is not reproduced so well, the amount of waste toner is also increased, so there is a considerable influence on the environmental load.

Therefore, in the printer according to the embodiment, the first current appropriate value I ₁ calculated using the average image area ratio X and the first formula for the 10 line sections passing through the Y primary transfer nip exit, The control unit 200 and the primary transfer power supply 81 are configured to perform a process of updating the output target value of the primary transfer current to the same value. The method for measuring the electrical resistance of the primary transfer roller 25Y will be described in detail later.

As can be seen from the second to fourth formulas, with respect to M, C, and K, the reverse transfer rate is kept at a certain value, so that the electrical resistance of the primary transfer roller is increased contrary to the first formula. and the first current is increased a proper value I ₁ with. In the second to fourth formulas, when the average image area ratio X is lower than 5%, the second appropriate current value I ₂ is rapidly decreased as the average image area ratio X decreases. This is because the aim is to minimize the occurrence of reverse transcription. Normally, when the primary transfer current is lowered, the transfer rate is reduced accordingly. However, when the average image area ratio X is lower than 5%, dots such as a character image instead of a solid image are used to some extent. In most cases, the image can be transferred with a smaller amount of current than a solid image. Further, if the average image area ratio X is low, even if the transfer ratio is lowered to some extent, it is difficult to visually recognize that the image density is insufficient. Therefore, with an emphasis on reverse transfer rate suppression than the transfer rate improvement, it is going to rapidly reduce the second current proper value I ₂ with decreasing average image area ratio X.

Further, when the average image area ratio X exceeds 95 [%], an average image area ratio of rapidly increasing second current proper value I ₂ with increasing X is due to the following reason. That is, when the average image area ratio X exceeds 95 [%], most of the area of the photoconductor entering the primary transfer nip carries a toner image, and there is almost no non-image portion. The reverse transfer of toner to the image portion hardly occurs. Therefore, than reverse transfer rate inhibition, with an emphasis on improving the transfer rate, it cause a rapid increase of the average image area ratio second current proper value I ₂ with increasing X. One of the reasons is that a high image density is required for an image that is almost solid.

  Therefore, in this printer, for the primary transfer nips for M, C, and K, the target value of the primary transfer current is obtained by a formula different from the primary transfer nip for Y. Specifically, in this printer, 5 [%] is adopted as an average image area ratio of a [%] that is a division between a large area background area and other areas (a = 5%). That is, the photosensitive region (10-line section) having an average image area ratio of less than 5% is handled as a large-area background region that is particularly susceptible to reverse transfer. Further, 95 [%] is adopted as the average image area ratio b [%] that is a division between the maximum image area and the other areas (b = 95%). In other words, a photoconductor area having an average image area ratio exceeding 95% is handled as a maximal image area close to the entire solid area. When the average image area ratio x is less than 5% (less than a%), that is, when the large area background area of the photoconductor is located at the exit of the primary transfer nip, the above equation 2 is used. A target value of the primary transfer current is obtained. That is, when the image area ratio is less than 5 [%] (less than a [%]), the output target value of the primary transfer current is set to a smaller value than when the average image area ratio x is in a general range. To do. Further, when the average image area ratio x is within a general range of 5 [%] to 95 [%], the target value of the primary transfer current is obtained based on the third equation. That is, when within the general range (a [%] to b [%] (where 0 <a <b), the output target value of the primary transfer current decreases as the average image area ratio x increases. When the average image area ratio x exceeds 95 [%], that is, when the maximum image area of the photosensitive member is located at the exit of the primary transfer nip, the primary transfer current is calculated based on the above-described fourth formula. In other words, when the average image area ratio x exceeds 95 [%] (when it exceeds b [%] (where b <100)), the average image area ratio x is within a general range. The output target value of the primary transfer current is set to a larger value than the time.

  FIG. 7 shows the case where the primary transfer roller used in the experiment is used in a configuration in which the primary transfer current is constantly controlled at a constant output target value as in a conventional general image forming apparatus. From the result, the output target value is generally set to about 22 [μA]. In such a configuration, when the average area ratio image X is relatively high, a significant transfer rate occurs. On the other hand, when the average image area ratio X is relatively low, the transfer ratio is remarkably lowered. Therefore, good image quality cannot be obtained. For example, when the output target value is 22 [μA], the primary transfer roller electrical resistance is 1.4E7, and the average image area ratio X is 95 [%], the reverse transfer rate is deteriorated to 5 [%]. Further, when the output target value is 22 [μA], the primary transfer roller electrical resistance is 1.4E7, and the average image area ratio X is 5 [%], the transfer rate is reduced to 91 [%]. When the output target value is 22 [μA], the primary transfer roller electrical resistance is 5.6E7, and the average image area ratio X is 5 [%], the reverse transfer rate is increased to about 3 [%], and The transfer rate decreases to about 92 [%].

  On the other hand, in the printer according to the embodiment, regardless of the average image area ratio X and the electrical resistance R, the transfer rate is 91 [%] to 96 [%] while the reverse transfer rate is suppressed to about 2 [%] or less. Can be maintained in the range.

The present inventors actually set the output target values of the primary transfer power sources 81M, C, and K to the same value as the _second appropriate current value I2 obtained under the conditions of the second to fourth formulas. When the control unit 200 and the primary transfer power sources 81M, C, and K are configured, the low reverse transfer rate and the high transfer rate are excellent regardless of the electric resistance R, particularly in a high area rate image including a lot of green and red. Image quality was achieved. Further, since the solid primary green and red supply a large primary transfer current in preference to the transfer rate, the image density can be considerably increased.

In the printer of the embodiment, the combination of the control unit 200 and the primary transfer power sources 81Y, 81M, 81C, 81C, 81K functions as transfer current output means. Then, the control unit 200 calculates the average image area ratio X of each 10-line section on the surface of the photoreceptor 2Y. Furthermore, for each 10 line sections, calculating an average image area ratio X, the electric resistance R of the primary transfer roller 25Y that is previously determined, based on the first equation, a first current proper value I _1. Then, the tip of the 10 lines compartment at the timing enters the primary transfer nip outlet for Y, a first current calculation result of the proper value I ₁ for the 10-line section, and outputs to the primary transfer power supply 81Y. The primary transfer power supply 81Y is an output target value to perform the Update constant current control to the first current proper value equal to the I ₁ sent from the control unit 200.

Further, the control unit 200 calculates the average image area ratio X of each 10-line section on the surface of the photoconductor 2M, C, and K, respectively. Further, for each 10-line section, the second current is determined based on the average image area ratio X, the electrical resistance R of the primary transfer rollers 25M, C, and K measured in advance, and the second to fourth formulas. to calculate the proper value _{I 2.} Then, M is the tip of the 10 line sections, C, at the timing enters the primary transfer nip outlet for K, a second current calculation result of the optimum value I ₂ for that 10 line sections, the primary transfer power supply 81M, Output to C and K. The primary transfer power supply 81M, C, K is the output target value, update the second current proper value equal to the I ₂ sent from the control unit 200 to implement a constant current control.

  In the printer according to the embodiment, the control unit 200, the primary transfer power supplies 81Y, M, C, and K, the resistance measurement rollers 27Y, M, C, and K, the relay switches 28Y, M, C, and K The combination of these functions as a resistance detection means. The resistance detection means is a primary transfer power supply in a state where resistance measuring rollers 27Y, M, C, K made of stainless steel (SUS304) having a diameter of 8 [mm] are grounded by the excitation of relay switches 28Y, M, C, K. A constant current is output from 81Y, M, C, K to the primary transfer rollers 25Y, 25M, 25C, 25K. Then, based on the output voltage value, the output current value, and Ohm's law from the primary transfer power supplies 81Y, M, C, and K at that time, the electrical resistance of the primary transfer rollers 25Y, M, C, and K Is calculated.

  The timing for calculating the electrical resistance R of the primary transfer rollers 25Y, 25M, 25C, 25K is at the start of the print job. When receiving the print command from the user, before starting the optical writing to the photoconductors of the respective colors, the primary transfer rollers 25Y, 25M, 25C, and 25K are driven while the photoconductors and the intermediate transfer belt are driven. The electrical resistance R is measured. Further, during the continuous printing operation, every time a predetermined number of prints are performed, the print job is temporarily interrupted, and the electric resistance R of the primary transfer rollers 25Y, 25M, 25C, 25K is measured. Thereby, not only the fluctuation of the electric resistance R caused by the deterioration of the primary transfer rollers 25Y, 25M, 25C, 25K but also the fluctuation of the electric resistance R of the primary transfer rollers 25Y, 25M, 25C, 25K due to the environmental change. Can also respond.

  In this printer having such a configuration, the appropriate value of the primary transfer current is based on both the average image area ratio X of 10 line sections on the photosensitive member at the exit of the primary transfer nip and the electrical resistance R of the primary transfer roller. Thus, the toner image is transferred at the primary transfer nip in a state where a transfer current having a more appropriate value is supplied as compared with the conventional case where the appropriate value is obtained by considering only the average image area ratio X. Thereby, generation | occurrence | production of image density nonuniformity can be suppressed compared with the past.

Note that the metal member brought into contact with the primary transfer rollers 25Y, 25M, 25C, 25K for measuring the electrical resistance is not limited to the roller shape. Brush-shaped or blade-shaped metal may be brought into contact with the primary transfer rollers 25Y, 25M, 25C, 25K, and the current may be guided to the ground. The material of the member is not limited to metal as long as it has good conductivity. However, the electrical resistance is required to be lower than that of the primary transfer roller. When employing a material such as a resin layer, it is desirable that the product of the thickness of the resin layer and the volume resistivity be 1E2 [Ωm ² ] or less.

  By the way, in the DTCC method, when the output target value of the primary transfer current is determined according to the average image area ratio at the transfer nip exit on the photosensitive member, the average image of the toner image on the intermediate transfer belt 21 at the transfer nip exit. The area ratio is not considered. This is for the reason explained below. That is, when a toner image is present on the intermediate transfer belt 21, the toner image on the intermediate transfer belt 21 does not take into account the average image area ratio, and the toner does not cause much reverse transfer. This is because the primary transfer voltage automatically increases.

  In the image forming apparatus described in Patent Document 2, the ratio of the overlap between the transferred toner image on the intermediate transfer belt and the pre-transfer toner image and the image ratio (from the sum of the transferred toner image and the pre-transfer toner image). Based on the value obtained by subtracting the overlapping portion), the output target value of the primary transfer current is determined. In such a configuration, a high transfer current is applied where the toners are not overlapped. As a result, the reverse transfer rate is increased, and a significant reduction in image density occurs. Further, the primary transfer current is increased at the time of overlap transfer, but the tendency of the transfer efficiency dependency curve of the transfer efficiency is the same between the single color and the overlap, and it is not necessary to increase the current at the time of overlap transfer. Furthermore, in order to determine the overlap, it is necessary to store print position information for each color in a storage medium such as a memory and calculate the overlap rate for each image, so a high-performance CPU and a large-capacity memory are required. It becomes. On the other hand, in this printer, since the transfer current is simply controlled according to the image area ratio of the toner image to be transferred, a high-quality image can be realized with an inexpensive system.

  Conventional primary transfer bias control methods include an ATVC (Active Transfer Voltage Control) method and a PTVC (Programmable Transfer Voltage Control) method in addition to a general constant current method and a general constant voltage method. It has been known. As an image forming apparatus adopting the ATVC method, an image forming apparatus described in JP-A-2-123385 is known. As an image forming apparatus adopting the PTVC method, an image forming apparatus described in JP-A-5-181373 is known.

  Both the conventional ATVC method and the PTVC method are basically constant voltage control for controlling the output current so that the output voltage becomes a predetermined output target value. When the resistance (electrical resistance value) of the primary transfer roller changes due to environmental fluctuations, the desired value of the output voltage changes accordingly. According to the result of measuring the resistance value of the primary transfer roller at every predetermined timing. Thus, it differs from general constant voltage control in that the output target value of the output voltage is changed. When measuring the resistance of the primary transfer roller, the current is controlled at a constant current in the ATVC method, whereas the constant voltage is controlled in the PTVC. In any of the methods, conventionally, the same value is adopted as the output target value of the output voltage for the first color and the second and subsequent colors, so that the toner on the belt is transferred to the photoreceptor at the primary transfer nip for the second and subsequent colors. Reverse transfer to the background. Further, the correction of the output target value of the output voltage based on the measured value of the resistance value of the primary transfer roller is performed at a predetermined timing. However, when the resistance value changes rapidly due to a sudden environmental change. The corrected value will not match the actual situation.

  On the other hand, since the DTCC method is a control that makes the current value constant, it can provide a stable output image even if the resistance of the primary transfer roller slightly changes. Further, the DTCC method taking into account the resistance of the primary transfer roller and the transfer nip of the present invention can provide a remarkably stable final image.

  As described above, when the primary transfer current in the primary transfer nips for M, C, and K is controlled by the DTCC method, reverse transfer occurs as the resistance of the primary transfer roller decreases. In order to suppress the above, an algorithm for lowering the primary transfer current as a whole is used. However, when the resistance of the primary transfer roller is remarkably lowered and the density reduction of the low image area ratio image of the solid image cannot be ignored, It is effective to increase the development voltage in conjunction with the decrease in primary transfer roller resistance. Control in this way is equivalent to no image density.

Next, the present inventor conducted the following experiment.
Three types of test images were output by a printer testing machine. As another one of the three types of test images, a K100 [%] test image (image area ratio 100 [%]) adhered to the entire surface of A4 size paper was printed. Also, as a printer, an M100 [%] + K5 [%] test image obtained by superimposing a K5 [%] test image on an M100 [%] test image that adheres to the entire surface of A4 size paper is printed. did. As with the K5 [%] test image, these test images are also gradually increased from 1000 [V] to 2300 [V] every 100 [V]. The primary transfer rate was measured. For the M100 [%] + K5 [%] test image, the amount of M toner adhered to the background portion of the photoreceptor 2K after passing through the primary transfer nip for K was measured, and the measurement result The ratio to the amount at the time of entering the nip was determined as the M toner reverse transfer rate. The toner adhesion amount was measured based on the spectroscopic measurement result by a reflection spectral densitometer X-Rite 938.

  FIG. 11 is a graph showing the relationship between the primary transfer voltage, the primary transfer current, and the test image in this experiment. FIG. 12 is a graph showing the relationship between the primary transfer rate, the primary transfer voltage, the M toner reverse transfer rate, and the test image in this experiment. FIG. 13 is a graph showing the relationship between the primary transfer rate, the primary transfer current, the M toner reverse transfer rate, and the test image in this experiment.

  As shown in FIG. 12, when a single-color toner image composed of only K is formed as a test image (K5 [%], K100 [%]), there is a primary transfer voltage regardless of the image area ratio. When the predetermined value is exceeded, the primary transfer rate starts to rapidly decrease. More specifically, when the primary transfer voltage exceeds 2000 [V], the primary transfer rate starts to rapidly decrease. Under the condition of 2000 [V], as shown in FIG. 11, the current flowing through the primary transfer nip varies depending on the image area ratio. Specifically, in the case of a K5 [%] test image, a primary transfer current of 30 [μA] is output from the primary transfer power supply 81K under the condition of primary transfer voltage = 2000 [V], whereas In the case of a K100 [%] test image, a primary transfer current of 21 [μA] is output from the primary transfer power supply 81K under the condition of primary transfer voltage = 2000 [V]. As described above, when the primary transfer bias is controlled at a constant voltage, the primary transfer current flows as the image area ratio on the photosensitive member decreases.

The reason for this is that, under the condition of constant voltage control in which the primary transfer voltage is controlled to be constant, the lower the image area ratio, the greater the charge amount of the photoreceptor, and the more current flows between the belt and the photoreceptor. Because it flows. For example, in a printer testing machine, the K photoconductor 2K is uniformly charged to about −500 [V] by a charging device. For the latent image portion (electrostatic latent image), the potential of −500 [V] is attenuated to about −30 [V] by irradiation with the laser beam L. Since the photoconductor 2K having a capacitance of 9.5E-7 [F / m ² ] is used, the area charge density of the background portion of the photoconductor 2K is about −475 [μC / m ² ]. Degree. On the other hand, the area charge density of the latent image portion of the photoreceptor 2K is the toner charge amount 0.45E-3 [g / cm ² ] × −20 [μC / g] = − 0.009 [μC / cm ² ] = -90 and [μC / ^{m 2],} since the sum of the residual potential of the photoreceptor charge amount (about -30 [V]) (-29μC / m 2), at about -119 [μC / ^{m 2]} is there. In the photoconductor 2K, the amount of charge in the background portion is about four times larger than that in the latent image portion. Therefore, the primary transfer nip is formed between the background portion of the photoreceptor 2K and the intermediate transfer belt 21 rather than the electric field formed between the latent image portion of the photoreceptor 2K and the intermediate transfer belt 21. The electric field is stronger. Then, as the image area ratio of the photosensitive member 2K decreases, the current easily flows between the belt and the photosensitive member. Therefore, the output current amount is set to set the voltage output value from the primary transfer power supply 81K to a predetermined value. It will increase.

  As described above, in the constant voltage control, the output current value from the power source increases as the image area ratio decreases. However, even if the image area ratio is the same, the output current value varies greatly depending on the environment. This is because when the environment changes, the resistance values of the intermediate transfer belt 21 and the primary transfer roller 25K change accordingly. Therefore, under constant voltage control conditions, even if the output target value of the output voltage is changed according to the image area ratio, the primary transfer current may become excessive or insufficient depending on the environment. May cause defects. For this reason, it is advantageous that the primary transfer bias is controlled not by constant voltage control but by constant current control. In addition, it is desirable to change the output target value of the output current according to the image area ratio, rather than simple constant current control.

  In the primary transfer nip for Y, it is desirable to adopt a value at which transfer efficiency as high as possible is obtained as the output target value of the primary transfer current for the reason described below. That is, the Y toner image sequentially passes through all of the primary transfer nips for M, C, and K, and each time, although slightly, the toner adheres to the photoreceptor and is lost. This is because they tend to be thinner than other color toner images. Therefore, for the output target value of the output current from the primary transfer power source 81Y for Y, it is desirable to increase the output voltage to a value at which the maximum transfer efficiency can be obtained. This experiment was performed in an environment of 25 [° C.], and the maximum transfer efficiency was obtained under the condition that the primary transfer voltage was 2000 [V]. Under this condition, a primary transfer current of 30 [μA] flows in the K5 [%] test image as shown in FIG. 11, whereas a primary transfer current of 21 [μA] flows in the K100 [%] test image. Flowing. In simple constant voltage control, when the room temperature changes from 25 [° C.] and the resistance of the belt or roller changes, the primary transfer current is excessive even if the primary transfer voltage is maintained at 2000 [V]. Become shortage. The value of 2000 [V] is a voltage condition for obtaining the maximum transfer efficiency in an environment of 25 [° C.]. When the room temperature is changed from 25 [° C.], the voltage condition for realizing the maximum transfer efficiency is also changed. Because it will end up. On the other hand, the current condition for obtaining the maximum transfer efficiency is constant regardless of the environment. Specifically, when the image area ratio is 5 [%], the maximum transfer efficiency can be realized by maintaining the primary transfer current value constant at 30 [μA] regardless of the environment. it can. When the image area ratio is 100 [%], the maximum transfer efficiency can be realized by maintaining the primary transfer current value constant at 100 = 21 [μA] regardless of the environment. .

  As described above, with regard to the primary transfer power supply 81Y for Y, the maximum transfer efficiency can be maintained by changing the output target value of the output current in accordance with the image area ratio. However, when the same constant current control is also performed with the M, C, K primary transfer power supplies 81M, C, K, the Y toner on the belt is transferred to the photosensitive member 2M in the M, C, K primary transfer nip. It was found that it was easy to reverse transfer to the background of C, K and K.

  For example, as shown in FIG. 12, in the primary transfer nip for K, depending on the value of the primary transfer voltage, the reverse transfer rate of the M100 [%] toner image on the belt to the photoreceptor 2K background portion ( M toner reverse transfer rate) becomes very high. Specifically, under the condition where the primary transfer voltage is set to 1000 to 1500 [V], the M toner reverse transfer rate remains below 0.01 [%], but the primary transfer voltage is from 1600 [V]. It can also be seen that the M toner reverse transfer rate starts to rise sharply when the value is also increased.

  On the other hand, when the primary transfer voltage exceeds 2000 [V], the reason why the transfer efficiency starts to rapidly decrease is considered as follows. That is, when the primary transfer voltage exceeds 2000 [V], the potential difference between the latent image portion of −30 [V] on the photoreceptor and the intermediate transfer belt 21 exceeds the discharge start voltage. Then, in the primary transfer nip, discharge is actively generated between the latent image portion (-30V) of the photoreceptor and the intermediate transfer belt 21, and the toner on the latent image portion is reversely charged by the discharge. End up. This reverse charging causes the toner on the latent image portion to remain on the latent image portion without being electrostatically moved onto the intermediate transfer belt 21, which is considered to be a cause of a decrease in transfer efficiency.

  When such a decrease in transfer efficiency occurs, not only between the −30 [V] latent image portion of the photoconductor and the belt but also −500 [V] of the photoconductor in the primary transfer nip. Electric discharge is also generated between the background portion and the belt. However, when printing a single color image, the toner image on the photoconductor is transferred to the intermediate transfer belt 21 in which no toner image exists in the primary transfer nip. No toner is interposed between the belt and the belt. For this reason, the discharge between the background portion and the belt does not appear as a prominent phenomenon. Paying attention to the latent image portion (-30V) of the photosensitive member in which a prominent phenomenon of a decrease in transfer efficiency appears, when the output voltage from the primary transfer power source is larger than 2000 [V], the latent image portion and belt of the photosensitive member The potential difference between the first and second voltages is made larger than the discharge start voltage. Since it is difficult to grasp the surface potential of the belt, when considering the output voltage from the primary transfer power supply for convenience, in this experiment, if the potential difference between the output voltage and the photosensitive member is larger than 2030 [V], This means that the potential difference between the photoconductor and the belt is larger than the discharge start voltage.

  As described above, in this experiment, when the output voltage from the primary transfer power supply 81K for K is larger than 1600 [V], the M toner reverse transfer rate starts to rapidly increase. The cause of such a rapid increase is considered as follows. That is, when printing a multicolor image by superimposing two or more colors instead of a single color image, the toner image already transferred onto the belt is transferred to the subsequent photoreceptor in the primary transfer nip for the second and subsequent colors. It is interposed between the background part of the belt and the belt. At this time, if the potential difference between the background portion (−500 V) of the photoconductor and the output voltage from the primary transfer power supply is larger than 2030 [V], discharge occurs between the background portion and the intermediate transfer belt 21. . Then, the toner in the toner image that has already been transferred onto the intermediate transfer belt 21 is reversely charged by the discharge and reversely transferred to the background portion of the photoreceptor. Since the potential of the background portion of the photoreceptor is about −500 [V], if the output voltage from the primary transfer power supply is larger than 1530 [V], such reverse transfer is caused. In this experiment, since the output voltage is increased in units of 100 [V], 1530 [V] corresponds to the condition of 1600 [V]. Therefore, in the graph shown in FIG. 11, when the primary transfer voltage exceeds 1600 [V], it is considered that the M toner reverse transfer rate starts to increase rapidly.

  Under the condition of room temperature 25 [° C.], as described above, the primary transfer current = 30 [μA] for the 5 [%] image and the primary transfer current = 21 [μA] for the 100 [%] image. In addition, the primary transfer voltage is about 2000 [V] at which the maximum transfer efficiency can be realized. It is assumed that such primary transfer current control is adopted not only by the primary transfer power supply 81Y for Y but also by the primary transfer power supplies 81M, C, and K for M, C, and K. Then, in the primary transfer nips for M, C, and K, the potential difference between the background portion of the photoconductor and the intermediate transfer belt is made larger than the discharge start voltage, so the toner on the belt is transferred to the photoconductor. It will be reverse transferred to the background.

  In the primary transfer nip for M, C, and K, when a discharge occurs between the background portion of the photoreceptor and the intermediate transfer belt 21 at the nip exit, the toner on the belt is reversed to a positive polarity opposite to the normal one. Charging and reverse transfer to the background of the photoreceptor. When the average image area ratio in the vicinity of the transfer nip exit of the photoconductor is below a predetermined a [%], that is, when a large area background area of the photoconductor enters the transfer nip exit, output based on the equation (1) When the target value is determined, the output value of the primary transfer bias from the primary transfer power supplies 81M, C, and K becomes very large. Then, the potential difference between the background portion of the photoreceptor and the belt at the exit of the transfer nip is made larger than the discharge start voltage, and it is easy to generate a discharge between them (see FIG. 12).

Therefore, in this printer, for the primary transfer nips for M, C, and K, the output target value of the primary transfer current is obtained as follows. 5 [%] is adopted as the average image area ratio a [%], which is a division between the large area background area and the other areas (a = 5 [%]). That is, the photosensitive region (10-line section) having an average image area ratio of less than 5% is handled as a large-area background region that is particularly susceptible to reverse transfer. Further, 95 [%] is adopted as the average image area ratio b [%] that is a division between the maximum image area and the other areas (b = 95 [%]). In other words, a photoconductor area having an average image area ratio exceeding 95% is handled as a maximal image area close to the entire solid area. Then, M, C, 1 transfer power supply 81M for K, C, for K, as already mentioned, based on the second equation to fourth equation, to determine a second current proper value _{I 2,} The control unit 200 is configured.

In FIG. 10 described above, focusing on the output target value of the primary transfer current for Y and the output target value of the primary transfer current for M, the average image is within an area ratio range of 95 [%] or less. If the area ratio is the same, the latter output target value is smaller than the former output target value. This is for the reason explained below. That is, the primary transfer nip for Y is always in the first transfer process, and since there is no toner image transferred on the belt before that, the toner from the belt to the background of the photoreceptor 2Y at the transfer nip exit. Does not cause reverse transcription. For this reason, the inclination and intercept of the graph are set focusing on obtaining excellent transfer efficiency. On the other hand, reverse transfer may occur in the primary transfer nips for M, C, and K. Therefore, in the primary transfer nips for M, C, and K, not only focusing on obtaining excellent transfer efficiency, but also focusing on suppressing toner reverse transfer, setting the slope and intercept of the graph doing. Therefore, in the range of 5 to 95 [%], if the average image area ratio is the same, towards the second current proper value I ₂ is than made smaller than the first current proper value I _1.

  Focusing on M, C, and K, the graph of the linear function has a negative slope when the average image area ratio is in the range of 5 to 95 [%]. On the other hand, when the average image area ratio is less than 5%, the linear function graph has a positive slope. This is for the reason explained below.

  That is, when the average image area ratio in the vicinity of the transfer nip exit of the photoconductor is extremely low, less than 5%, there is almost no toner on the photoconductor at the transfer nip exit. In particular, when the average image area ratio is 0 [%], there is no toner on the photoconductor at the transfer nip exit. In such a state, there is no need to transfer toner from the photoreceptor to the belt. Therefore, basically, even if the application of the primary transfer bias is interrupted, the image quality is not affected. Further, even if it is 0 [%] or more, under the condition of a very low average image area ratio of less than 5 [%], an image with a minimum area is located at the transfer nip exit. When the color tone of this minimal area image is not halftone, it is mostly a character image or a line image. In the case of a character image or a line image, even if some toner reverse transfer occurs, image density unevenness due to the toner is hardly visually recognized. Further, when the color tone of the image of the minimum area is a halftone, the isolated dot-like dot latent images surrounded by the background portion are arranged at predetermined intervals on the surface of the photoreceptor. In this state, a very small dot latent image is surrounded by the background portion, and the primary transfer current of the background portion flows into the latent image portion due to the positive charge of the dot latent image and the potential of the surrounding background portion. It becomes easy. Moreover, since the amount of toner adhering to the dot latent image is very small, the toner on the dot latent image can be transferred to the intermediate transfer belt 21 without increasing the primary transfer voltage so much. Further, if attention is paid to the waste toner amounts of all colors Y, M, C, and K, the reverse transfer toner amount by reducing the primary transfer current rather than the increase in transfer efficiency by increasing the primary transfer current. It is advantageous to prioritize the reduction of This is because giving priority to the reduction in the amount of reverse transfer toner by reducing the primary transfer current can reduce the waste toner and increase the effective utilization rate of the toner. As a matter of course, when the image area ratio of Y toner becomes larger, the output value of the primary transfer voltage from the M primary transfer power supply 81M increases, and the reverse transfer ratio of Y toner increases. . For this reason, when the average image area ratio at the transfer nip exit on the photoconductor is remarkably low as in this printer, it can be seen that lowering the current is more advantageous in reducing the amount of waste toner.

  Therefore, when the average image area ratio in the vicinity of the nip exit of the photoconductor is less than 5% in the primary transfer nips for M, C, and K, the output target value of the primary transfer current is made sufficiently small. Effectively suppress the occurrence of reverse transcription. From the viewpoint of avoiding the occurrence of reverse transfer, it is desirable to reduce the output target value to around 10 [μA]. However, in such a case, the output of the power source is output even though the primary transfer current has to be rapidly increased when the photosensitive region having an average image area ratio of about 10% enters the exit of the transfer nip. It cannot be sufficiently increased from the limit of responsiveness, and there is a risk of causing a transfer failure. Therefore, 15 [μA] or more is maintained without lowering to 10 [μA]. Depending on the responsiveness of the power supply, it may be lowered to about 10 [μA]. In addition, in the range where the average image area ratio is less than 5 [%], the graph has a positive slope because the average of the next moment as the average image area ratio approaches 5 [%] from 0 [%]. This is because there is a high possibility that the image area ratio becomes slightly larger than 5 [%] and the primary transfer current must be rapidly increased to about 26 [μm].

  At the M, C, and K primary transfer nip exit, if the average image area ratio of the 10 line sections on the photoconductor is 100 [%], the toner adheres to all the image formable areas of the photoconductor. As a result, the photosensitive member background portion does not exist at the transfer nip exit. At the transfer nip exit, the toner on the belt is reversely transferred to the background of the photoconductor, so that reverse transfer does not occur in the absence of the photoconductor background. Therefore, even if the primary transfer current is very large, reverse transfer does not occur. In addition, the entire solid image is positioned at the transfer nip exit, and unlike the halftone image, the entire solid image tends to have noticeable unevenness and insufficient density of the transferred image due to insufficient transfer current. In particular, when the pressure unevenness at the primary transfer nip is large between the rear and front of the apparatus (left and right on the output image), it is easily recognized as a left-right density deviation. Therefore, when the average image area ratio is 100 [%], it is desirable to prioritize good transfer efficiency by setting the primary transfer current output target value higher than usual.

  If the average image area ratio is not 100 [%] but exceeds 95 [%], the primary transfer current output target value should be set higher than usual to give priority to good transfer efficiency. Is desirable. Therefore, the fourth formula is adopted. As shown in FIG. 10, the graph having a negative slope in the range of 5 to 95 [%] turns to a positive slope in the range exceeding 95 [%]. As the average image area ratio increases, the output target value of the primary transfer current also increases. As a result, the primary transfer current can be made higher than usual to suppress the occurrence of insufficient image density.

  As previously shown in FIG. 10, in each of M, C, and K, the output target value of the primary transfer current determined when the average image area ratio is 0 [%] is the average image area ratio. It is set smaller than the output target value determined when it is 100 [%]. Accordingly, priority is given to suppression of reverse transfer when 0 [%], while improvement of transfer efficiency can be prioritized when 100 [%].

Next, modifications of the printer according to the embodiment will be described. Unless otherwise specified, the configuration of the printer according to each modification is the same as that of the embodiment.
[First Modification]
In the printer according to the first modification, instead of the resistance detection means, an electrical characteristic detection means for detecting an electrical characteristic correlated with the electrical resistance of the primary transfer rollers 25Y, M, C, K is provided. . The electrical characteristic detecting means rotates the primary transfer rollers 25Y, 25M, 25C, 25K, and the photoconductors 2Y, M, C, and K, and moves the intermediate transfer belt 21 endlessly. A constant current is supplied from M, C, K to the primary transfer rollers 25Y, M, C, K. The IV characteristic, which is the relationship between the output current value from the primary transfer power supply 81Y, M, C, and K and the output voltage at this time, is obtained as an electrical characteristic.

  FIG. 14 is a graph showing IV characteristics of a primary transfer roller having an electric resistance R of 1.4E7 [Ω]. FIG. 15 is a graph showing IV characteristics of the primary transfer roller having an electric resistance R of 5.64E7 [Ω]. In experiments conducted by the present inventors, the nip resistance Rnip, which is the electrical resistance of the primary transfer nip, can be obtained from these IV characteristics as follows. That is, the photosensitive member is uniformly charged at −520 [V], and the entire surface of the photosensitive member is moved to the primary transfer nip with the non-image portion (background portion), and the primary transfer roller has 30 μA. The relationship between the output voltage Vout [V] from the primary transfer power supplies 81Y, M, C, and K when the current is supplied and the nip resistance Rnip is expressed by the equation "Rnip = 4.94E4V-1.32E7". . Hereinafter, this equation is referred to as a fifth equation. The nip resistance Rnip is a combined resistance of the primary transfer roller and the intermediate transfer belt.

It has been confirmed by experiments of the present inventor that the nip resistance Rnip is close to the electric resistance R of the primary transfer rollers 25Y, 25M, 25C, 25K measured based on Ohm's law. As the first equation to fourth equation described above, built on a formula for the first current proper value I ₁ and the second current proper value I ₂ based on the nip resistor Rnip an average image area ratio X in the experiment by it, as in the case of measuring the electrical resistance R, it is possible to determine the appropriate values as the first current proper value I ₁ and the second current proper value I _2.

When the electrical resistance of the intermediate transfer belt 21 is sufficiently lower than the electrical resistance of the primary transfer rollers 25Y, 25M, 25C, 25K, the nip resistance Rnip obtained by the fifth equation is used as it is as the primary transfer roller 25Y. , M, C, may be obtained an electric resistance first current as R are substituted into the first formula to fourth formula proper value I ₁ and the second current proper value I ₂ for K. For example, in the printer testing machine used in the experiment, when the primary transfer rollers 25Y, 25M, 25C, 25K have an electrical resistance of R = 1.4E7 [Ω], the volume resistivity of the intermediate transfer belt 21 is x. Even when a material having [Ω · cm] or a material having a volume resistivity of 2 × [Ω · cm] was used, the IV characteristics hardly changed. In the printer testing machine, it can be considered that the ratio of the electrical resistance of the intermediate transfer belt 21 to the transfer nip resistance Rnip is 1/10 or less, so that the nip resistance Rnip is regarded as almost the electrical resistance R. There is no problem.

  In the printer according to the first modification, the nip resistance Rnip at the primary transfer nip for Y, M, C, and K is measured as follows. That is, the surface of the intermediate transfer belt 21 is roughly divided into a sheet corresponding region that is superimposed on the recording sheet at the secondary transfer nip and a sheet non-corresponding region that is not superimposed on the recording sheet in the circumferential direction. Further, the sheet non-corresponding area includes a sheet non-operating area immediately after the start or end of a print job, and an inter-sheet corresponding area positioned between two adjacent sheet corresponding areas during the continuous image forming operation. being classified. The electrical characteristic detecting means determines the nip resistance Rnip of the primary transfer nip for Y, M, C, K when the sheet non-corresponding region has not entered the primary transfer nip for Y, M, C, K. taking measurement. When the continuous printing operation is being performed, the nip resistance Rnip of the primary transfer nip for Y, M, C, and K each time the corresponding area between sheets enters the primary transfer nip for Y, M, C, and K. Measure. Thereby, even if the temperature and humidity in the apparatus fluctuate during the continuous printing operation, it is possible to cope with fluctuations in the electrical resistance R of the primary transfer roller caused by the fluctuations.

[Second Modification]
The printer according to the second modified example has the same configuration as that of the printer according to the embodiment except for the following points.
As shown in FIG. 9, when a Y solid image having an average image area ratio x of 100 [%] (1.0) is primarily transferred, the first transfer is performed regardless of the electrical resistance of the primary transfer current. current proper value _{I 1} is substantially a constant value (about 3.0E-0.5A). The primary transfer current having this constant value is measured in advance by experiments (hereinafter, the measured value is referred to as an appropriate current value for all solid transfer). The average image area ratio x is 100% under the condition of constant current control in which the target value of the output current value from the primary transfer power source 81Y for Y is set to the same value as the appropriate current value for all solid transfer. An output voltage value from the primary transfer power supply 81Y during the primary transfer of the Y test solid image from the Y photoconductor 2Y to the intermediate transfer belt 21 is measured (hereinafter referred to as an output voltage value during full solid transfer). . The measured value is the primary resistance required to form a primary transfer electric field having an appropriate strength between the primary transfer roller 25Y and the photoreceptor 2Y under the condition of the electrical resistance of the primary transfer roller 25Y at that time. This is the potential of the core of the transfer roller 25Y. If the electrical resistance of the primary transfer roller 25Y does not change, the potential of the core metal of the primary transfer roller 25Y is completely solid regardless of the average image area ratio x Y toner image. If the value is the same as the output voltage value during transfer, a primary transfer electric field having an appropriate strength can be formed between the primary transfer roller 25Y and the photoreceptor 2Y.

Therefore, under the condition that the output from the primary transfer power supply 81Y is controlled at a constant voltage with the output voltage value at the time of full solid transfer as a target value, the entire background portion of the photoreceptor 2Y is used as the primary transfer nip for Y. The output current value from the primary transfer power supply 81Y when it is entering is measured (hereinafter, this measured value is referred to as the output current value when entering the entire background portion). Then, the output current value at the time of entering the entire background portion is set to the first current appropriate value I ₁ when the average image area ratio x = 0 [%], and the appropriate current value at the time of full solid transfer is set to the average image area ratio x = 100 [ %] Request a linear function of the linear equation to the first current proper value I ₁ when the. Then, during the print job, the first appropriate current value I ₁ corresponding to the average image area ratio x of the Y 10 line section is calculated, and the output target value is calculated from the primary transfer power supply 81Y in the constant current control. Update to the same value.

As described above, for the most upstream Y, as the electrical characteristics correlated with the electrical resistance of the primary transfer roller 25Y, the output voltage value at the time of full solid transfer and the output current value at the time of entering the entire background portion are measured. , based on a linear function was constructed based on the measurement results, obtaining the first current proper value I _1.

As shown in FIG. 10, when primary transfer of M, C, K solid images having an average image area ratio x of 100 [%] (1.0) is performed regardless of the electrical resistance of the primary transfer current. not, the second current proper value _{I 2} is substantially a constant value (about 2.0E-0.5A). This value is measured in advance and stored in the control unit as an appropriate current value for M, C, and K solid transfer. Under the condition of constant current control in which the target value of the output current value from the primary transfer power supply 81M, C, K for M, C, K is the same value as the appropriate current value for M, C, K solid transfer, 1 during the primary transfer of M, C, K test solid images having an average image area ratio x of 100 [%] from the M, C, K photoconductors 2M, C, K to the intermediate transfer belt 21 The output voltage value from the next transfer power supply 81M, C, K (hereinafter referred to as the output voltage value at the time of M, C, K all solid transfer) is measured. Each measured value has an appropriate strength between the primary transfer rollers 25M, C, K and the photoreceptors 2M, C, K under the conditions of the electrical resistance of the primary transfer rollers 25M, C, K at that time. This is the potential of the core metal of the primary transfer rollers 25M, C, K required to form the primary transfer electric field. If the electrical resistance of the primary transfer rollers 25M, C, and K does not change, the primary transfer roller 25M can be used for primary transfer of M, C, and K toner images of any average image area ratio x. , C, K cores are set to the same value as the output voltage value during M, C, K solid transfer, between the primary transfer rollers 25M, C, K and the photoreceptors 2M, C, K. A primary transfer electric field having an appropriate strength can be formed.

  Therefore, the entire surface of the photoreceptors 2M, C, K is controlled under the condition that the output from the primary transfer power supply 81M, C, K is constant voltage controlled with the output voltage value at the time of all solid transfer of M, C, K as a target value. The output current value from the primary transfer power supply 81M, C, K when the portion of the background portion is entered into the primary transfer nip for M, C, K is measured (hereinafter, this measured value is referred to as M, C , K is called the output current value when entering the entire background.

  In addition, the average of the photoreceptors 2M, C, and K under the condition that the output from the primary transfer power supply 81M, C, and K is constant voltage controlled with the output voltage value at the time of all solid transfer of M, C, and K as a target value. The output current value from the primary transfer power supply 81M, C, K when the portion where the image area ratio x is 95 [%] is entered into the primary transfer nip for M, C, K is measured (hereinafter referred to as “the current transfer ratio”). This measured value is called M, C, K95% output current value when entering the image portion).

  In addition, the average of the photoreceptors 2M, C, and K under the condition that the output from the primary transfer power supply 81M, C, and K is constant voltage controlled with the output voltage value at the time of all solid transfer of M, C, and K as a target value. The output current value from the primary transfer power sources 81M, C, and K when the portion where the image area ratio x is 5 [%] is entered into the primary transfer nip for M, C, and K is measured (hereinafter referred to as the following). These measured values are referred to as M, C, K 5% output current value when entering the image portion).

Then, for each of M, C, and K, a general range in which the average image area ratio x is 5 to 95% based on the 5% image portion entry output current value and the 95% image portion entry output current value. In this case, a linear line is obtained. Specifically, 5% image area entry when the output current value and second current proper value I ₂ when the average image area ratio x = 5 [%], and 95% image portion enters when the output current value average image A linear function is obtained as a second current appropriate value I ₂ when the area ratio x = 95 [%].

Next, based on the output current value at the time of entering the 5% image portion and the output current value at the time of entering the 95% image portion, a linear line when the average image area ratio x is in a general range of 5 to 95%. Ask for. Specifically, 5% image area entry when the output current value and second current proper value I ₂ when the average image area ratio x = 5 [%], and 95% image portion enters when the output current value average image A linear function is obtained as a second current appropriate value I ₂ when the area ratio x = 95 [%]. In the print job, when the average image area ratio x of the 10 line sections is in a general range, the output target value of the primary transfer current is updated based on this linear function.

Moreover, 95% of image portion enters when the output current value and second current proper value I ₂ when the average image area ratio x = 95 [%], and K the total time of solid transfer mean a proper current value image area ratio x = A linear function that is a second current appropriate value I _{2 at} 100 [%] is obtained. In the print job, in order for the average image area ratio x of 10 line sections to exceed 95 [%], the output target value of the primary transfer current is updated based on this linear function.

Also, the second current proper value I ₂ at the time of the entire background portion enters when the output current average image area ratio x = 0 [%], and 5% image portion enters when the output current average image area ratio x = A linear function that is the second current appropriate value I _{2 at} 5 [%] is obtained. When the average image area ratio x of the 10 line sections is less than 5% during the print job, the output target value of the primary transfer current is updated based on this linear function.

As described above, for M, C, and K, as the electrical characteristics correlated with the electrical resistance of the primary transfer roller 25Y, the output voltage value at the time of full solid transfer, the output current value at the time of full surface entry, and 5 The output current value at the time of entering the% image part and the output current value at the time of entering the 95% image part are measured, and the second appropriate current value I ₂ is obtained based on a linear function constructed based on the measurement result.

In such a configuration, it is possible to obtain the primary transfer rollers 25Y, M, C, respectively for K, without obtaining an electric resistance R and the nip resistance Rnip, the first current proper value _{I 1} and the second current proper value _{I 2} .

[Third Modification]
FIG. 16 is a schematic configuration diagram illustrating a printer according to a third modification. This printer does not have a re-transmission device, the recording sheet conveyance direction in the apparatus is the direction from the lower side to the upper side in the vertical direction, and the image forming units for each color are disposed below the intermediate transfer belt 21. Is different from the printer according to the embodiment. By not providing a retransmission device, double-sided printing cannot be performed, but the size of the device is reduced accordingly.

[Fourth Modification]
FIG. 17 is a schematic configuration diagram illustrating a printer according to a fourth modification. This printer is different from the printer according to the embodiment in that, as a belt member, an endless paper conveyance belt 121 is brought into contact with each color photoconductor 2Y, M, C, K instead of the intermediate transfer belt. ing. The paper transport belt 121 sequentially passes the recording paper held on its surface through the primary transfer nips for Y, M, C, and K along with its endless movement. In this process, Y, M, C, and K toner images on the photoreceptors 2Y, 2M, 2C, and 2K are transferred onto the surface of the recording paper in a superimposed manner.

[Fifth Modification]
FIG. 18 is a schematic configuration diagram illustrating a printer according to a fifth modification. This printer has Y, M, C, and K developing devices 3Y, 3M, 3C, and 3K around one photoconductor 2. When performing image formation, first, the surface of the photoreceptor 2 is uniformly charged by the charging unit 4, and then the laser light L modulated based on the Y image data is applied to the surface of the photoreceptor 2. Irradiation forms an electrostatic latent image for Y on the surface of the photoreceptor 2. The Y electrostatic latent image is developed by the developing device 3Y to obtain a Y toner image, which is then primarily transferred onto the intermediate transfer belt 21. Thereafter, the transfer residual toner on the surface of the photoreceptor 2 is removed by the drum cleaning device 5, and then the surface of the photoreceptor 2 is uniformly charged again by the charging unit 4. Next, the surface of the photoconductor 2 is irradiated with laser light L modulated based on the image data for M to form an electrostatic latent image for M on the surface of the photoconductor 2. Is developed by the developing device 3M to obtain an M toner image. Then, the M toner image is primary-transferred superimposed on the Y toner image on the intermediate transfer belt 21. Thereafter, in the same manner, the C toner image and the K toner image are sequentially developed on the photosensitive member 2, and are sequentially superposed on the YM toner image on the belt for primary transfer. As a result, a four-color toner image is formed on the intermediate transfer belt 21.

  Thereafter, the four-color toner image on the intermediate transfer belt 21 is secondarily transferred onto the surface of the recording paper at the secondary transfer nip to form a full-color image on the recording paper. Then, after the full color image is fixed on the recording paper by the fixing device 40, the recording paper is discharged out of the apparatus.

  In such a configuration in which the superposition transfer is performed by the circular method, the same algorithm as that for Y in the embodiment is used in the transfer process of the first color (first round). On the other hand, in the transfer process for the second and subsequent colors (second to fourth rounds), the same algorithm as that for M in the embodiment is used.

What has been described above is merely an example, and the present invention has a specific effect for each of the following modes.
[Aspect A]
Aspect A is a latent image carrier (for example, photoconductors 2Y, 2M, 2C, and 2K) that carries a latent image, and developing means (for example, a developer) that develops the latent image on the latent image carrier with toner. Devices 3Y, 3M, 3C, and 3K) and contact members (for example, primary transfer rollers 25Y, 25M, 25C, and 25K) that contact the latent image carrier directly or via a belt member (for example, intermediate transfer belt 21). A recording sheet in which the toner image on the latent image carrier is sandwiched in the transfer nip at the transfer nip by the contact between the latent image carrier and the contact member, or the contact between the latent image carrier and the belt member. Alternatively, a transfer current is output to the contact member for transfer to the surface of the belt member, and a transfer current is output each time the surface of the latent image carrier is sent to the transfer nip by a predetermined length. The target value is based on the image area ratio in the predetermined length range In an image forming apparatus provided with transfer current output means (for example, control unit 200 and primary transfer power supplies 81Y, 81M, 81C, 81K) that updates to the same value as the measured appropriate value, the electrical resistance of the contact member is detected In addition to providing a resistance detection means for detecting a predetermined electrical characteristic correlated with the electrical resistance, the appropriate value is added to the image area ratio, and the resistance detection means or the electrical characteristic is provided. The transfer current output means is configured so as to perform the processing to be obtained based on the detection result by the detection means.

[Aspect B]
Aspect B provides a plurality of combinations of latent image carriers and developing means in Aspect A, and provides a plurality of contact members so as to contact the latent image carriers via belt members, respectively, and move In the process of sequentially passing the belt member through a plurality of transfer nips by a plurality of latent image carriers, the toner image on the latent image carrier is held on the belt member or the surface of the belt member at each transfer nip. It is characterized in that the recording sheet is superimposed and transferred. In such a configuration, the toner images formed substantially in parallel to the plurality of latent image carriers are superposed, so that the plurality of toner images are superposed compared to the case where the toner images are formed at individual timings and then superposed. A toner image can be formed in a short time.

[Aspect C]
Aspect C uses an endless belt member as in Aspect A, and the toner image on the latent image carrier is transferred to the surface of the belt member in the process of revolving the belt member and repeatedly passing through the transfer nip. It is characterized in that the image is superimposed and transferred. In such a configuration, a superimposed toner image can be formed even if only one latent image carrier is provided.

[Aspect D]
In the aspect D, in the aspect B or C, as an algorithm for obtaining the appropriate value, among the plurality of transfer processes for superimposing transfer, in the most upstream transfer process, the detection result is on the high resistance side. While the output target value is decreased as the value is shifted to the value, the output target value is increased as the detection result shifts to a value on the high resistance side for at least one of the remaining transfer processes. The transfer current output means is configured so as to use. In this configuration, as described above, in the most upstream transfer process that does not generate reverse transfer, the toner image is transferred under the current condition where the improvement of the transfer rate is given the highest priority, while reverse transfer is generated. In the transfer process that can be performed, the toner image can be transferred under a current condition in consideration of the balance between the improvement of the transfer rate and the suppression of the reverse transfer rate.

[Aspect E]
Aspect E is characterized in that, in aspect D, the transfer current output means configured to detect the relationship between the current value and the output voltage value when a constant transfer current is output to the contact member as the electrical characteristics, It is made to function as a detection means. In such a configuration, it is possible to grasp the electric resistance of the transfer current without providing a metal member to be brought into contact with the contact member or a relay switch for turning on and off between the metal member and the ground.

[Aspect F]
The aspect F is the aspect of the aspect E, wherein the transfer current configured to calculate a combined resistance between the contact member and the belt member based on the relationship and to obtain the appropriate value based on the calculation result. The output means functions as the resistance detection means. In such a configuration, it is possible to grasp the electric resistance of the transfer current without providing a metal member to be brought into contact with the contact member or a relay switch for turning on and off between the metal member and the ground.

[Aspect G]
Aspect G is the aspect E or F in which the relation is reached at the timing when the image forming operation is being performed and all the areas entering the transfer nip in the entire area of the latent image carrier become non-image portions. The transfer current detecting means is configured so as to carry out the process of measuring. In this configuration, as described above, the target value of the transfer current can be updated to an appropriate value even if there is an environmental change during the continuous image forming operation.

[Aspect H]
Aspect H outputs the predetermined second transfer current when the predetermined first transfer current is output while moving the surface of the latent image carrier and the belt member during the non-image forming operation in aspect F. The transfer current detection means is configured to measure the relationship at each of the above and perform processing for obtaining the appropriate value based on the measurement results. In such a configuration, the electrical resistance of the abutting member can be grasped more accurately than when the transfer current value is fixed to only one.

[Aspect I]
In the aspect I, in the state F, the latent image carrier and the belt member are moved on the surface and a predetermined transfer toner image is output while a user does not accept an image forming operation command. On the latent image carrier, the relationship when the first image area ratio portion of the test toner image enters the transfer nip, and the second image area ratio of the test toner image. The transfer current detection unit is configured to perform the process of calculating the combined resistance based on the above when the portion is entering the transfer nip. In such a configuration, the electrical resistance of the contact member can be grasped more accurately than in the case where the value of the image area ratio at the time of voltage measurement is fixed to only one.

[Aspect J]
Aspect J is the algorithm for obtaining the appropriate value in any one of aspects B to I, with respect to the remaining transfer processes excluding the most upstream transfer process among a plurality of transfer processes for superimposing transfer. When the image area ratio is within a general range of a [%] to b [%] (where 0 <a <b), the output target value is decreased as the image area ratio increases. On the other hand, when the image area ratio is less than a [%], the transfer current output means is used so that the output target value is smaller than that when the image area ratio is in a general range. It is characterized by comprising. With this configuration, it is possible to suppress the occurrence of reverse transfer of toner to the latent image carrier at the transfer nip exit. Specifically, in the remaining transfer process, reverse transfer of toner is particularly likely to occur when the large area background area of the latent image carrier is positioned at the transfer nip exit. The large area background area is an area where the image area ratio at the transfer nip exit is a very small value of 0% or more and less than a [%] (for example, less than 5%) in the entire area of the latent image carrier. is there. When the image area ratio = 0 [%], it is not necessary to execute the transfer process in the first place. Even if the image area ratio is not 0 [%], the amount of toner transferred from the latent image carrier to the recording sheet or belt member is extremely small when the image area ratio is very low, less than a [%]. Or the current flows in from the surrounding maximum area of the latent image portion of the latent image carrier, so even if a large transfer current is not output, the latent image carrier can be minimized. A necessary amount of current can be supplied to the latent image portion of the area. Therefore, when the image area ratio is less than a [%], that is, when the reverse transfer is particularly likely to occur, the target value of the transfer current is made smaller than when the image area ratio = a [%]. To do. As a result, the toner is transferred by passing a necessary amount of transfer current to the minimal latent image portion where the image area ratio is less than a [%], and the background portion of the latent image carrier and the nip forming member at the transfer nip exit And the occurrence of reverse toner transfer can be suppressed.

[Aspect K]
In the aspect K, when the image area ratio exceeds b [%] in the remaining transfer processes excluding the most upstream transfer process among a plurality of transfer processes for superimposing transfer as an algorithm in the aspect J (Where b <100), and the transfer current output means is configured to use an output target value larger than that when the image area ratio is within a general range. It is. In this configuration, as described above, when transferring a full-color image or an image with a higher image area ratio as it is closer to this, even if the primary transfer current is very large, the belt to the photoconductor is transferred. Since the reverse transfer of the toner hardly occurs, the primary transfer current can be made very large to effectively suppress the image density shortage.

2Y, M, C, K: photoconductor (latent image carrier)
3Y, M, C, K: Developing device (developing means)
21: Intermediate transfer belt (belt member)
25Y, M, C, K: primary transfer roller (contact member)
81Y, M, C, K: primary transfer power supply (part of transfer current output means and resistance detection means)
200: Control unit (part of transfer current output means and resistance detection means)
P: Recording paper (recording member)

JP-A-8-83006 JP 2003-186284 A

Claims (11)

A latent image carrier that carries a latent image; a developing unit that develops the latent image on the latent image carrier with toner to obtain a toner image; and a contact member that is in contact with the latent image carrier directly or via a belt member. A toner image on the latent image carrier at a transfer nip formed by contact between the contact member, the latent image carrier and the contact member, or contact between the latent image carrier and the belt member; A transfer current is output to the contact member for transferring to the recording sheet sandwiched between the transfer nips or the surface of the belt member, and the surface of the latent image carrier is transferred to the transfer nip. An image forming apparatus comprising: a transfer current output unit that updates an output target value of the transfer current to the same value as an appropriate value obtained based on an image area ratio in the predetermined length range each time a predetermined length is sent In
While providing a resistance detection means for detecting the electrical resistance of the contact member, or an electrical characteristic detection means for detecting a predetermined electrical characteristic correlated with the electrical resistance,
The transfer current output unit is configured to perform processing for obtaining the appropriate value based on a detection result by the resistance detection unit or the electrical characteristic detection unit in addition to the image area ratio. Image forming apparatus. The image forming apparatus according to claim 1.
While providing a plurality of combinations of the latent image carrier and the developing means,
A plurality of the abutting members are provided so as to abut against the latent image carriers via the belt members,
and,
In the process of sequentially passing the moving belt member through a plurality of transfer nips by a plurality of latent image carriers, a toner image on the latent image carrier is transferred to the belt member or the surface of the belt member at each transfer nip. An image forming apparatus, wherein the image is transferred in a superimposed manner on a recording sheet to be held. The image forming apparatus according to claim 1.
As the belt member, an endless one is used,
In the process of rotating the belt member around and repeatedly passing through the transfer nip, the toner image on the latent image carrier is transferred onto the surface of the belt member in an overlapping manner. apparatus. The image forming apparatus according to claim 2 or 3,
As an algorithm for obtaining the appropriate value, among the plurality of transfer processes for superimposing transfer, for the most upstream transfer process, as the detection result shifts to a value on the high resistance side, the output target value While at least one of the remaining transfer steps, the transfer current output is increased so that the output target value is increased as the detection result shifts to a value on the high resistance side. An image forming apparatus comprising a means. The image forming apparatus according to claim 4.
The transfer current output means configured to detect a relationship between a current value and an output voltage value when a constant transfer current is output to the abutting member as the electrical characteristic is caused to function as the electrical characteristic detection means. An image forming apparatus. The image forming apparatus according to claim 5.
The transfer current output means configured to calculate a combined resistance of the contact member and the belt member based on the relationship and obtain the appropriate value based on the calculation result is the resistance detection means. An image forming apparatus characterized by functioning as an image forming apparatus. The image forming apparatus according to claim 5 or 6,
During the image forming operation and out of the entire area of the latent image carrier, the process of measuring the relationship is performed at the timing when all the areas entering the transfer nip become non-image parts. An image forming apparatus comprising the transfer current detecting means. The image forming apparatus according to claim 6.
During the non-image forming operation, the relationship is different between when the predetermined first transfer current is output and when the predetermined second transfer current is output while moving the surface of the latent image carrier and the belt member. An image forming apparatus, wherein the transfer current detecting means is configured to perform a process of measuring and obtaining the appropriate value based on the measurement result. The image forming apparatus according to claim 6.
Forming a predetermined test toner image on the latent image carrier while moving the surface of the latent image carrier and the belt member and outputting a predetermined transfer current in a state where an image forming operation command by the user is not received. The relationship when the first image area ratio portion of the test toner image enters the transfer nip and the second image area ratio portion of the test toner image enter the transfer nip. An image forming apparatus comprising: the transfer current detecting unit configured to perform the process of calculating the combined resistance based on the above. The image forming apparatus according to any one of claims 2 to 9,
As an algorithm for obtaining the appropriate value, among the plurality of transfer processes for superimposing transfer, the remaining transfer processes excluding the most upstream transfer process have an image area ratio of a [%] to b [ %] (Where 0 <a <b), the output target value is decreased as the image area ratio increases, while the image area ratio is less than a [%]. An image forming apparatus, wherein the transfer current output means is configured to use an output target value that is smaller than that when the image area ratio is in a general range. The image forming apparatus according to claim 10.
As the algorithm, when the image area ratio exceeds b [%] in the remaining transfer processes excluding the most upstream transfer process among a plurality of transfer processes for superimposing transfer (however, b <100) An image forming apparatus, wherein the transfer current output means is configured to use an output target value larger than that when the image area ratio is in a general range.

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